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SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 



k 



Hocument coHtla^^NWormation affecting the national defense of the 
nited States within thelftiMiiQg of the Espionage Act, 50 U. S. C., 31 and 32, 
amended. Its transnnssio5Ni^^hft.4:evelation 
an unauthorized person is ' ' 


This volume is classified 
tions of the War and 

material which was^p m^FJ.LtJt.v r<,^ at the date ot printing. 
may have had a low^^lassification or none. The reader is advised to 


Departments because ^ 
fF-f IJJV'N' TTM i at the date of printing. 


i^h security regula- 
rs contain 
rs 
1 


the War and N^;y^gencies listed on the reverse of this page for the currem 
classification offiiw material. 


2SSiEIEiS?li^ 


The present volume was originally prepared by Central Com- 
munications Research, Cruft Laboratory, Harvard University, as 
Part II of the final report on Contract OEMsr-1441. Front and 
rear matter, prepared by the Summary Reports Group of the 
Columbia University Division of War Research under contract 
OEMsr-1131 with the Office of Scientific Research and Develop- 
ment, has been printed by and the volume bound by the Colum- 
bia University Press. 

Distribution of the Summary Technical Report of NDRC has been 
made by the War and Navy Departments. Inquiries concerning the 
availability and distribution of the Summary Technical Report 
volumes and microfilmed and other reference material should be 
addressed to the War Department Library, Room lA-522, The 
Pentagon, Washington 25, D. C., or to the Office of Naval Re- 
search, Navy Department, Attention: Reports and Documents 
Section, Washington 25, D. C. 


Copy No. 


This volume, like the seventy others of the Summary Technical 
Report of NDRC, has been written, edited, and printed under 
great pressure. Inevitably there are errors which have slipped past 
Division readers and proofreaders. There may be errors of fact not 
known at time of printing. The author has not been able to follow 
through his writing to the final page proof. 

Please report errors to : 

JOINT RESEARCH AND: DEVELOPMENT BOARD 

programs division (str errata) 

WASHINGTON 25 , D. C. • 

• H v- * '. n .. ' ' 

A master errata $heet ^yill be compiled from these reports and sent 
to recipients of the volume. Your help will make this book more 
useful to other readers and will be of great value in preparing any 
revisions. 


SUMMARY TECHNICAL REPORT OF DIVISmN 13, NDRC 

^ ®"‘=retary of 

2 3 mo 


VOLUME 2B 


-^cfeiise 


2 




ELECTRONIC 


NAVIGATION SYSTEMS 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 


DIVISION 13 


HARADEN PRATT. CHIEF 




WASHINGTON, D. C,, 1946 


NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative^ 

Frank B. Jewett Navy Representative ^ 

Karl T. Compton Commissioner of Patents* 

Irvin Stewart, Executive Secretary 


^ Army representatives in order of service: 


^Navy representatives in order of service: 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen. W. A. Wood, Jr. 

Col. E. 


Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
A. Routheau 


Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
* Commissioners of Patents in order of service: 

Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facihties for carrying out 
these projects and programs, and (2) to administer the tech- 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research projects on re- 
quests from the Army or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the work involved in such projects were 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra- 
tion of patent matters were handled by the Executive Sec- 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A — Armor and Ordnance 

Division B — Bombs, Fuels, Gases, & Chemical Problems 
Division C — Communication and Transportation 
Division D — Detection, Controls, and Instruments 
Division E — Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three ad- 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the particular field. The NDRC members then be- 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 

Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Explosion 

Division 3 — Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface Warfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 — Absorbents and Aerosols 

Division 11 — Chemical Engineering 

Division 12 — Transportation 

Division 13 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 17 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


iv 



lyibrary (if (’(>iigress 



201.S 


490942 


NDRC FOREWORD 


A S EVENTS of the years preceding 1940 revealed 
more and more clearly the seriousness of the world 
situation, many scientists in this country came to 
reahze the need of organizing scientific research for 
service in a national emergency. Recommendations 
which they made to the White House were given 
careful and sympathetic attention, and as a result the 
National Defense Research Committee [NDRC] was 
formed by Executive Order of the President in the 
summer of 1940. The members of NDRC, appointed 
by the President, were instructed to supplement the 
work of the Army and the Navy in the development 
of the instrumentalities of war. A year later, upon the 
establishment of the Offiee of Scientific Research and 
Development [OSRD], NDRC became one of its 
units. 

The Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum- 
marize and evaluate its work and to present it in a 
useful and permanent form. It comprises some 
seventy volumes broken into groups corresponding to 
the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the work 
of that group. The first volume of each group’s report 
contains a summary of the report, stating the prob- 
lems presented and the philosophy of attacking them, 
and summarizing the results of the research, develop- 
ment, and training activities undertaken. Some 
volumes may be “state of the art” treatises covering 
subjects to which various research groups have con- 
tributed information. Others may contain descrip- 
tions of devices developed in the laboratories. A 
master index of all these divisional, panel, and com- 
mittee reports which together constitute the Sum- 
mary Technical Report of NDRC is contained in a 
separate volume, which also includes the index of a 
microfilm record of pertinent technical laboratory 
reports and reference material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of 
sufficient popular interest that it was found desirable 
to report them in the form of monographs, such as the 
series on radar by Division 14 and the monograph on 
sampling inspection by the Applied Mathematics 
Panel. Since the material treated in them is not dupli- 
cated in the Summary Technical Report of NDRC, 








■<^ary 


23 


Weo 


the monographs are an import^icfi^a^^of^e'sl/ifi^?^^' 
of these aspects of NDRC research. - 

In contrast to the information on radar, which is oi ^ 
vddespread interest and much of which is released 
to the public, the research on subsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consequence, the report of 
Division 6 is found almost entirely in its Summary 
Technical Report, which runs to over twenty volumes. 

The extent of the work of a division cannot therefore 
be judged solely by the number of volumes devoted 
to it in the Summary Technical Report of NDRC: 
account must be taken of the monographs and avail- 
able reports published elsewhere. 

Of all the NDRC Divisions, few were larger or 
charged with more diverse responsibilities than 
Division 13. Under the urgent pressure of wartime 
requirements, the staff of the Division developed 
navigation and communications devices and systems 
which not only contributed to the successful Allied 
war effort, but which will continue to be of value in 
time of peace in the fields of transportation and com- 
munications. The work of the Division, under the 
direction first of C. B. Jolliffe and later of Haraden 
Pratt, furnishes a foundation for what promises to be 
even more radical developments than those of the 
war — for one example, direction finders which will 
operate at all elevations and azimuth angles, in other 
words, hemispherically. 

The Summary Technical Report of Division 13 was 
prepared under the direction of the Division Chief 
and authorized by him for publication. The report 
presents the methods and results of the widely varied 
research and development program, and, in the case 
of work with speech scrambling and decoding, it 
presents for the first time a comprehensive review of 
the state of the art. The report is also a notable 
record of the skill and integrity of the scientists and 
engineers, who, with the cooperation of the Army and 
Navy and Division contractors, contributed bril- 
liantly to the defense of the nation. To all of these we 
express our sincere appreciation. 

Vannevar Bush, Director 
Office of Scientific Research and Development 
J. B. CoNANT, Chairman 
National Defense Research Committee 



9 






FOREWORD 


T he present volume, Volume 2B of Division 13, 
NDRC, was originally prepared by Central 
Communications Research, Cruft Laboratory, Har- 
vard University, as Part II of the final report on 
Contract OEMsr-1441, Service Project AN-31. It is 
included in this series as a supplement to Volume 2 A 
of the Summary Technical Report of Division 13, 
NDRC. It presents a descriptive and critical survey 
of a number of the electronic navigation systems de- 
veloped during the war, both in this country and in 
Europe. Some of the systems described are now or 
were operational during the war. Others are merely 
proposed. Pin-point bombing and blind landing 
techniques are excluded. 

Section 1 represents an analysis of the principles 
imderlying the three basic types of navigation systems 


together with notes on the accuracies and limitations 
inherent in them. 

Sections 2 through 30 embody descriptions more or 
less detailed of the various systems. Section 31 con- 
tains comparisons of the several systems, conclusions 
as to their characteristics, a table summarizing nu- 
merical and other data, and recommendations for 
further research. 

The report (OSRD Report No. 6279) of which 
this volume is a reprint was dated December 1, 1945, 
and, necessarily, the survey work leading to this re- 
port was completed sometime before that date. Thus, 
systems proposed after the end of 1945 are not in- 
cluded. 

Haraden Pratt 
Chief, Division 13 




vii 









J 


•T 




CONTENTS 


SECTION PAGE 

1 Introduction 1.01 

2 Beacons and Interrogators 2.01 

3 Oboe 3.01 

4 Shoran 4.01 

5 Micro-H 5.01 

6 A.R.L. Intermittent Phase-Comparison Distance- 

Measuring System 6.01 

7 A.R.L.One-Shot Distance-Measuring System .... 7.01 

8 Canadian Distance-Measuring System 8.01 

9 GE Random Interrogation Distance-Measuring System 9.01 

10 GE Time-Rationing Distance-Measuring System . . . 10.01 

11 Gee 11.01 

12 Loran 12.01 

13 Decca Navigational System (QM) 13.01 

14 Post-Office Position Indicator (POPI) 14.01 

15 A-N Radio ^^Range'’ 15.01 

16 Aircraft Direction-Finders and Homing Systems . . . 16.01 

17 Sonne (Consol) (AN/FRN-5) 17.01 

18 Bendix Automatic Position-Plotter 18.01 

19 CAA VHF Omnidirectional Beacon 19.01 

20 CAA LF Omnidirectional Beacon 20.01 

21 Federal Long-Range Navigational System 21.01 

22 Airborne Radar 22.01 

23 Search Radar as a Navigational Aid 23.01 

24 AN/APN-34 Short-Range Approach System .... 24.01 

25 Federal Airport Traffic-Control System 25.01 

26 Map-PPI Superposition (Radar Mapping) 26.01 

27 RCA Television-Radar System 27.01 

28 Sperry Omnidirectional-Range and Distance Indicator . 28.01 

29 AN/APA-44 Ground-Position Indicator (GPI) . . . 29.01 

30 Miscellaneous Enemy Navigational Systems .... 30.01 

31 Comparisons, Conclusions and Recommendations . . 31.01 

32 Appendix A — A Short Glossary of Terms Used in this 

Report 32.01 




CONTENTS 


SECTION PAGE 

33 Appendix B — Probable Values of some Physical and 

Geodesic Constants 33.01 

34 List of Microfilmed Reports 34.01 

35 Contract Numbers 35.01 

36 Service Project Numbers 36.01 

37 Index 37.01 


Electronic Navigation Systems 


1.01 


Introduction 


Under the pressure of wartime requirements a great many new and useful 
navigational methods have been developed, planned, or suggested. The purpose of 
this survey is to make a basic study of all known radio aids to navigation in order 
to set up fair and useful bases of comparison, and in order to provide a common 
background for further developmental programs. Conveniently, all known systems 
may be analyzed in terms of three basic types, which are described and criticized 
in Section 1 of this report. 

In Sections 2 to 30 inclusive, we have attempted to summarize available 
knowledge regarding specific systems, showing how each system is built up by 
dressing one or more of the three basic types with instruments appropriate to the 
occasion. In part, this instrumentation is determined by the particular navigational 
problems which the designers of the system have attempted to meet. In part, the 
choice of particular instruments and expedients seems arbitrary, depending upon 
individual preferences and previous achievements of the system engineers. In some 
such instances, we have mentioned alternative methods or devices, especially when 
the alternative procedure seems to offer certain advantages. However, the authors 
of this report specifically disclaim any personal experience or specialized back- 
ground in electronic navigation, other than that which they have acquired in this 
research by analyzing all available documents, visiting a number of research labor- 
atories, and conferring repeatedly with the various Army, Navy, and Coast Guard 
groups in charge of such developments and applications. This fresh approach to 
the problem has permitted a strictly impartial consideration of all systems. If any 
personal preferences occasionally are implied in the text, it should, therefore, be 
understood that such thoughts have developed in the course of this survey, and now 
represent the consensus of opinion of an unbiased five- man jury. 

The authors consider an important part of their contribution to be the col- 
lection and presentation of descriptions of the numerous systems, existing or pro- 
posed. If these systems are to be critically compared, such a collection of descrip- 
tive material would in any case have to be made, and this report therefore presents 
both the conclusions reached and an outline of the knowledge on which they are 
based. Collection of these data proved to be a major enterprise, requiring the selec- 
tion and analysis of several hundred documents, picked out from thousands of ab- 
stracts of related material. Where possible these documents were supplemented 
and brought up-to-date by personal inquiries and by inspection trips. After "thresh- 
ing out" each system at some meeting of our research group, individual authors 
have described the several systems assigned to them, attempting to observe a 
reasonably uniform practice in text and diagrams. Such descriptions normally be- 
gin with a brief summary in a standardized tabular form, intelligible to any scienti- 
fically-minded reader. This introduction is followed by a presentation of such tech- 
nical features of each system as are deemed likely to interest persons who are 
reasonably familiar with electronic circuitry in general. Familiar items such as 
conventional transmitters and receivers have been treated very lightly, in order to 
conserve space for a fuller presentation of important novelties in electronic tim- 
ing devices , phasing networks , etc . Particular attention has been paid to those tech- 
nical assets and liabilities of each system which seem likely to determine its ulti- 
mate precision and coverage. The descriptive sections are intentionally very un- 
equal in length. Considerable detail has been included in treating systems which 
have been placed in actual operation. Some of the proposed systems are very in- 
teresting and may later prove to be valuable, but their components are somewhat 
nebulous at this stage in the design and can be treated adequately in a few sentences. 


1.02 


Electronic Navigation Systems 


With one very minor exception we have had complete access to all pertinent 
information available in the United States and England. This information naturally 
included data regarding a considerable number of enemy systems, though we have 
had to describe some of these in outline form only, lacking details of instrumenta- 
tion (in October 1945) at the time our survey closed. The gaps are probably not 
important, as the one outstanding German system is covered in considerable detail, 
partly on the basis of post-war inspection of significant components. On account of 
the classification assigned to most of our source material at the time when such 
information was obtained, we find it obligatory to assign the initial classification of 
"Secret" to this survey. However, we hope and expect that our initial classification 
may soon be reduced or deleted by competent governmental authority. Such action 
would release the information to a larger group of engineers at a time when it 
should be particularly useful in planning a coordinated world- wide navigation sys- 
tem. At the specific suggestion of the Army and Navy, we have always included the 
post-war aspects of the problem in our debates, in addition to the closely-related 
military tasks. In our own judgment the descriptions and diagrams here included 
are suitable for declassification at this time. 

In order to conserve space we have excluded from our descriptions certain 
highly specialized problems which the term "navigation" might otherwise include. 
The specific exclusions are: 1. Pin-point bombing 

2. Blind landing 

3. Electronic Altimeters. 

Accordingly, our treatment of "navigation" includes the location of military 
vehicles and troops on land, the navigation of ships at sea, and the guidance of air- 
craft over long distances and in the neighborhood of airports. Pilotage of ships 
under difficult conditions is included in our field of interest, but our problem is sus- 
pended while an aircraft is making its "bombing run" in the immediate vicinity of 
a hostile objective and terminates when an aircraft arrives at the "stacking space" 
over a friendly airport. These restrictions accord with the original directives in 
which the survey was requested. 

One additional comment on the word "navigation" may be useful by way of 
introduction to our subject. In the broad sense "navigation" is typified by the pro- 
blem of the tramp steamer, which should be able to follow any arbitrary route, any- 
where on the high seas. We do not speak of "navigation" when referring to the loco- 
motive engineer, who follows a steel track and is governed by block signals, warn- 
ing torpedoes, etc. Commercial airlines, especially over land, present an inter- 
mediate case, lying between these two extremes. Normally the pilot follows an 
electronic track, equipped with radio beacons and equivalent milestones. However, 
he may inadvertently become a "navigator" in the broader sense, in case he is 
driven "off -the -beam" by severe storms or other emergencies. Modern versions 
of the "beam" system are strongly favored by the airlines and are included in this 
report, though our main interest attaches to the more difficult problems of general 
unrestrifCted navigation. 

Following the numerous descriptions of specific systems we have included 
a few final remarks (Section 31) on comparisons, evaluations, conclusions, and 
suggestions for continued research and development. For the most part, these are 
the natural and almost inevitable comparisons and suggestions which germinate 
automatically when a small discussion group submerges itself in such a welter of 
related systems for a number of weeks. Constant restraint has been necessary in 
order to keep individual members of the group from inventing a dozen or more new 
variants of the species already on display. Scattered at appropriate places through- 
out the descriptive sections will be found a few minor researches, of a theoretical 


Electronic Navigation Systems 


1.03 


nature, aimed at evaluation of the capabilities and limitations of the general prin- 
ciples appearing in the context. 

We wish to acknowledge the cordial cooperation of the Army and Navy Liai- 
son Officers associated with the project and with o\ir laboratory. Great assistance 
has been rendered by the 06RD Liaison Office in Washington which assisted in the 
procurement of the majority of the foreign and domestic documents employed in 
the survey. Exchange of information with the Watson Laboratories and the Wright 
Field research group of the Army Air Force, and with Division 14 of 06RD has been 
particularly valuable. Provided with suitable credentials, we have been hospitably 
received by a number of the industrial laboratories that are represented in sections 
which follow. If exception be taken to any of our evaluations of the industrial pro- 
posals, we are confident that our sincerity and good faith will not be questioned, 
however much we may be criticized on scientific principles and applications. 

Insome cases, diagrams and descriptive material have been freely borrowed 
from the appropriate documents, which are listed in the bibliography at the end of 
each section. In other cases, where the wealth of available material made consider- 
able condensation necessary, a new approach has been used. The documents listed 
in each bibliography do not, of course, represent all the available references, but 
rather those which we have found to be of greatest value for our purpose. 

Space does not permit a complete description of the detailed circuitry in each 
of the radar systems contained in this report. The circuits of one radar system 
(AN/APS-1 5) are described in detail. Descriptions of the other radar systems omit 
detailed circuitry and consider only the general characteristics, important features, 
and special techniques involved. The circuit details may be found in various refer- 
ences listed under each system. 

Table 1.01 serves as an outline for Section 1. It will also be repeated, with 
appropriate insertions, at the beginning of each of the descriptions of individual sys- 
tems ; the table containing a summary of the characteristics of the particular sys- 
tem under examination. Finally these individual tables are collected on a single 
sheet (pages 31.06, 31.07). 


1.04 


Electronic Navigation Systems 


J. 

Table 1.01 


1 . Type of System (How the information is obtained) ' " 

(a) Pure range system (circular lines of position) » 

(b) Differential range system (hyperbolic lines of position) “ 

(c) Azimuth system (radial lines of position) 

2r Useful Range (Over what coverage area the information is obtainable) 

(a) Dependence on transmitter power ^ " 

(b) Other considerations t ^ ‘ ^ 

3. Accuracy and Precision (Errors in the results obtained) ' ^ i i v ' ^ 

^ — - — “TT ^ ^ , ? 't . - 

(a) The time measurement. Maximum theoretical precision obtainable with 

equipment ’ t 

(b) The propagation path.* Uncertainties due to propagation 

(c) Ambiguities ^ 

(d) Geometrical considerations in obtaining a "fix" from two or more lines of 

position " 

4. Presentation or Use of Data (How the inf ormation is presented to the navigator) 

(a) Aural ' 

(b) Visual ' ^ 

(c) Automatic control 

5. Operating Skills Required (Training and experience) ^ - 

(a) At ground or fixed installation ^ ^ ' 

(b) In the navigated craft 

(c) Time required to obtain a line of position or a fix 

6. Equipment Required (Weight, complexity, service and maintenance requirements) 

(a) At ground or fixed installation 

(b) In the navigated craft 

7. Radio-frequency Spectrum Allotments Required (Frequency, wavelength, band- 
width) 


8. Present Status of Development 


Electronic Navigation Systems 


1.05 


I. Types of Navigation Systems 


a. Pure range systems 

All systems depend on the constancy of the velocity of electromagnetic waves. 
This quantity relates the measurement of a time or an electrical phase angle to the 
determination of a distance or an angle in space. Consider first the determination 
of a distance which is measured by measuring the time of travel of an electro- mag- 
netic disturbance over the distance. The simplest application of this fundamental 
idea is that of radar "ranging" or distance measuring. (The word "simplest" used 
above does not refer to the equipment used to make the measurement but rather to 
the simplicity of the governing principle). The radar transmitter sends out a sig- 
nal which may be a pulse-modulated or otherwise modulated radio frequency car- 
rier. This transmission travels outwardfrom the transmitter with a spherical wave 
front. Aircraft, surface vessels, obstructions, and geographical or tropospheric 
discontinuities and variations in the dielectric constant all reflect as "echoes" a 
small part of the transmitted energy. A sensitive receiver located at or near the 
transmitter receives and amplifies these "echoes". By suitable circuitry the elaps- 
ed time between the sending of the signal and its return as an echo is measured 
and the distance to the reflector of the echo is then known. Suppose for example 
that such a measurement has been made and turns out to be twenty miles and that 
the reflector in this case is a surface vessel. The radar operator would then know 
that somewhere at a distance of twenty miles from his set (the position of which is 
assumed known) there is a surface vessel. (The operator may also get some measure- 
ment of the azimuth angle to the particular vessel but that is another part of the story, 
so let us suppose for the moment that all he knows is the distance). He could then 
draw a circle of twenty- mile radius on his chart, and thus establish one line of posi- 
tion for the surface vessel. A similar determination on the same vessel from a radar 
set at a different location would establish a second circular line of position. These 
two circular lines of position drawn on a chart about their respective transmitter 
locations will intersect at two points, one of which corresponds to the location of the 
surface vessel. The "fix" might equally well be determined by taking radar range 
measurements from a craft to two or more known fixed reflectors or responding 
beacons . In any case this pure range type of system is characterized by circular lines 
of position. The responding beacons referred to above are simply receiver-transmit- 
ters which send back an amplified "echo" or response, thus extending the useful range 
of the system and perhaps identifying the point from which the response was returned. 

It is implicit in the pure range or distance measurement type of system that 
the position of the navigated craft is disclosed in the general case where the craft 
is to have its "fix" determined. Either it must radiate a signal to man-made reflec- 
tors or beacons or to geographical reflectors. Or if its position is determined by 
radar methods from fixed stations it must be informed of its fix. This information 
may be coded or returned on a narrow beam communication system, televised, or 
otherwise kept secret. It is however possible to "vector" or direct a craft by ground 
radar determinations without exposing the position of the craft. The direct range 
system has the inherent property of being saturable. A system is said to be satur- 
able when the number of craft which may navigate with the aid of a given fixed instal- 
lation is limited. Thus the number of aircraft or surface vessels which may be 
"tracked" by a single radar set is limited, as is the number of craft which can inter- 
rogate a given beacon at any one time without overloading it. If the radar set is on 
the craft, then the number of such craft which can radiate signals in a given band is 
limited by interference. Either each craft must send out pulses at his particular 
frequency so that he can distinguish them in echo, or each must interrogate a beacon 
at the beacon receiving frequency and match his own characteristic pulse rate in 
order to select his own response. In the first case more craft mean more channels 


1.06 


Electronic Navigation Systems 


in the spectrum, in the second the number of responses which a beacon can put out 
is limited by overlapping. Practically, the theoretical condition of overlapping is 
not approached since average power output for present day beacons limits the per- 
missible number of responses per unit time for a given pulse energy. If one attempts 
to shorten pulses to avoid overlapping, the result is increased band width of the 
spectrum required for the transmission. The fundamental reason for the saturation 
effect is the "round trip" nature of the transmission. This in turn is necessary if 
one is to measure total time of travel. There is at present no sufficiently accurate 
absolute time clock with a microsecond "hand". If one had such a clock aboard the 
craft and knew that a certain station transmitted a pulse every second on the even 
second on precise Greenwich time, he could then measure the time that the pulse re- 
quired to travel from the fixed station to his position, and hence have a direct measure 
of distance with one way transmission, and hence no saturation effect or disclosure 
of position. 

At the present time it is possible to build crystal oscillators which under pro- 
per operating conditions, temperature, voltage control, etc., will run with an uncer- 
tainty not exceeding 1 part in a thousand million. 1 part in 10^ is equivalent to 1 
microsecond in 17 minutes or 5.37 microseconds in 1.5 hours which is an uncertainty 
of 1 mile in 1.5 hours. (5.37 microseconds is the time required for light or radio 
waves to travel one mile.) Thus having set the high precision clock described above, 
a navigator could determine his position with an uncertainty of one mile or less for 
a time of one hour and a half from the setting of his clock. This means that at the 
present state of the art one might navigate by pure range measurement up to 450 
miles at a speed of 300 miles per hour, without saturation or disclosure of his posi- 
tion with a maximum error of 1 mile in the determination of his line of position, 
assuming of course that the above accuracy could be maintained with airborne equip- 
ment as well as at the ground station. This hypothetical pure range system requires 
the accurate measurement of very long time intervals which is not yet practicable. 
Shorter intervals of time can be measured more exactly, that is to say an extremely 
accurate clock is required to measure with a precision of one microsecond in 1000 
secondsbut it is easily possible to measure time with a precision of one microsecond 
in one second. The microsecond is a very convenient unit of time and because of 
the constancy of the velocity of light it is quite common to speak of distances in terms 
of microseconds, i.e. one statute mile is equivalent to 5.37 microseconds for a one- 
way trip, or to 10.74 microseconds if a round-trip path is considered as in radar 
echoes. A number of useful conversions from distance to time and corresponding 
wave lengths and frequencies are given in Appendix B at the end of this document. 

b. Differential range systems 

In the differential range systems, a shorter time interval is measured to deter- 
mine a line of position. This is accomplished by transmitting signals from two fixed 
antennas, these signals having a known time or phase difference as they leave the 
transmitting antennas. If the signals are pulses, these are separated by a time inter- 
val known to the navigator who is to use the system. If they are modulated or key- 
ed then the modulation envelopes or radio frequency cycles have a known phase re- 
lationship. Suppose for example, that two transmitting stations are located at points 
A and Bin Figure 1-01 where the distance A to B is known as the base, and is in this 
example 111.72 miles long, that distance being chosen since it corresponds to six 
hvmdred microseconds time of travel of a radio wave from A to B. Suppose that 
pulses are sent out simultaneously from A and B so that a navigator with proper 
measuring equipment could determine the difference in the time of arrival of these 
pulses at his craft. If the craft were at any point on the line which bisects the base 
line at right angles, then the two pulses would arrive simultaneously, or conversely 
if the navigator received the pulses simultaneously he would know that he was some- 
where on the perpendicular bisector of the base line. If he received the pulse from 
B 100 microseconds before the pulse from A he would know that he was 18.62 miles 


Electronic Navigation Systems 


1.07 



Fig. 1-01 Construction of hyperbolic lines of position 


1.08 


Electronic Navigation Systems 


nearer B than A. The locus of points which are 18.62 miles nearer B than Amay 
be found by arbitrarily selecting a distance and drawing a circle of this arbitrary 
radius about point B; and then drawing a circle about A which has a radius 18.62 
miles greater than that about B. The locus of points thus determined is the curve 
shown in Figure 1-01, (solid line), which is the line of position for the craft when 
the measurement indicates that the pulse from B arrives 100 microseconds be- 
fore the pulse from A. Figure 1-02 shows two families of concentric circles 
drawn (dashed lines) about points A and B, the successive radii of the circles 
being 18.62 miles or 100 microseconds apart. Connecting successive inter- 



Fig. 1-02 Family of hyperbolic position lines, showing the relationship to an optical 

interference pattern 

sections of circles (solid lines) gives lines of position corresponding to time 
differences of 100 up to 600 microseconds difference in time of arrival of the 
two pulses. These lines of position are hyperbolae (if one assumes that the earth 
is flat, which is very approximately true for coverage areas not more than 500 
miles across) since they are the loci of points whose distances to A are greater 
than their corresponding distances to B by a constant amount. Navigation systems 
of this type are called hyperbolic or differential range systems. In the region to 
the left of the base-line bisector in Figure 1-01 and 1-02 the pulse from A will 
arrive first and one could draw a similar family of lines of position. The navigator 
must be able to distinguish the A pulse from the B pulse in order to tell whether 
he is to the right or left of the bisector. If the pulses are indistinguishable there 
will bean ambiguity. This is avoided in certain systems by delaying the pulse from 
one station by a time greater than the time to traverse the base line length. If pulse 
A is so delayed then the pulse from B will arrive first at all points in the diagram, 
there will be no ambiguity. Suppose that pulse A is delayed 700 microseconds be- 
hind pulse B . Figure 1-03 then shows the same diagram as Figure 1-02 without 


Electronic Navigation Systems 


1.09 




Fig. 1*03 Family of hyperbolic lines of position, with the A pulse delayed 700 
microseconds behind the B pulse 


1.10 


Electronic Navigation Systems 


the construction circles and with the hyperbolae marked with appropriate delay 
times in microseconds. Two or more such sets of hyperbolic lines of position will 
be necessary for determination of a "fix". Note that with direct range measurement 
one transmitting antenna could define a family of Circular lines of position, whereas 
with the differential range system at least two transmitting antennas are necessary 
to define a family of hyperbolic lines of position. With the differential range system 
the craft emits no signals of its own and hence does not disclose itis position, and 
furthermore the system is not saturable. The longest total time which the navigator 
must be able to measure is the total delay time between the sending out of pulses 
plus the time- length of base line. Since the time-length of the base line entered into 
the delay time as a necessary minimum value when there is to be no ambiguity, the 
total time to be measured may be somewhat longer than twice the length of the base 
line expressed as a time. In Figure 1-03 this is 1300 microseconds for points atthe 
extreme right of the diagram. Thus in general the greater the length of the base line 
the longer the time interval which the system must be capable of measuring. The 
only serious limitation on the length of the base line is that synchronism must be 
maintainedbetweenstations whose pulses are to bear a fixed relation to one another, 
and this in turn means that the pulse from one must be receivable at the other under 
all propagation conditions . Very long base lines are desirable for hyperbolic systems, 
since the lines of position will then be more or less straight and parallel over a 
large area; this in turn means a large area over which the maximum precision of 
the system is attainable. As pointed out earlier, the lines of position of a hyperbolic 
system are hyperbolae if the curvature of the earth can be neglected. However, as 
a number of people have pointed out, if one could use the longest possible base line, 
which is half the circumference of the earth, and set up transmitters at each pole, 
then the lines of position would be circular and would in fact be parallels of latitude. 
Hyperbolic systems lend themselves to fixed course navigation where the fixed course 
is a hyperbola. It is interesting to note that when navigating on a hyperbolic course 
the tangent to the course at the position of the craft always bisects the angle made 
bylines drawn from the position of the craft to each of the two stations which define 
the hyperbola. This in turn means that the vector components of velocity towards 
each station are equal in magnitude. 

Pulse transmissions have to be repeated with a known repetition pattern, 
usually at a constant repetition rate. As long as the repetition period is somewhat 
greater than twice the longest time to be measured there will be no ambiguity. On 
the other hand it is desirable to keep repetition rates fast enough so that the navigat- 
ed craft does not move too far between nieasurements. If the measurement of time 
is made visually on an oscilloscope then it is desirable to have the repetition rate 
fast enough to avoid flicker. In the example of a pulse- modulated system just consi- 
dered a delay time of 100 microseconds was added to the time length of the base to 
make up the total delay time of the A pulse. This delay time has two useful func- 
tions, First, it avoids the need of measuring very short time intervals which may be 
difficult in a system designed for measuring long intervals. Second, it may be varied 
in a predetermined manner for denying the effective use of the system to the enemy. 

As a second example of a hyperbolic system suppose that the transmissions 
from stations A and B of Figure 1-04 are sinusoidally modulated at 833 cycles per 
second, so chosen because the wavelength corresponding to 833 cycles per second 
is twice the base length which is 600 microseconds, and that the radio frequency of 
stations A and B is different so that the transmissions from the two stations are 
recognizable. Suppose further that the modulation envelopes are 180^ out of phase 
as they leave the transmitting antennas. On the craft being navigated there are two 
receiving circuits tuned to the transmissions from A and B, and a suitable means of 
phase comparison (it is assumed that equal phase-shifts are introduced by the two 
receivers). Onthebisector of thebase line the two modulation envelopes from A and 


Electronic Navigation Systems 


1.11 




Fig. 1-04 Hyperbolic position lines, similar to Fig. 1-03, but with delay times de- 
fined in terms of phase differences between the A and B signals. Base line = a/2 


1.12 


Electronic Navigation Systems 




1-04, but with base line =? X 


Electronic Navigation Systems. 


1.13 


B will be delayed in transmission by the same amount and will arrive, as they were 
transmitted, 180® out of phase. Along the base-line extension from the A station 
the phase difference between the two envelopes will be zero, if one uses the A trans- 
mission as phase zero, and the position lines are lines of constant time difference 
as they were in Figures 1-02 and 1-03 except that the time is measured in terms of 
phase angles of the 833-cycle modulation envelope. The period corresponding to 833 

cycles is = 1200 microseconds, and a phase difference of 30® at 833 cps corres- 


ponds to a time-difference of 100 microseconds. Note that for this choice of modula- 
tion frequency there is no ambiguity and also that 360 electrical degrees of phase 
shift corresponds to 180® of azimuth angle about the center of the base line. If a 
higher modulation frequency is used, the ratio of electrical to azimuthal degrees is 
increased, but ambiguities arise. For instance, suppose the modulation frequency is 
1666 cps so that the wavelength of the modulation cycle is equal to the base line 
length, and that the modulation envelopes are in phase as they leave the transmitting 
stations. Figure 1-05 then shows the resultant phase relationship between the receiv- 
ed A and B modulation envelopes. It is evident that there are two lines of position 
corresponding to any measured phase angle between 0 and 360® and hence an ambi- 
guity. However, if the phase control of the transmitted modulation envelopes and 
the precision of phase measurement at the cf'aft in terms of degrees of electrical 
phase angle is the same at 1666 cps as it was at 833 cps, the precision of a line of 
position will be twice as high at the higher frequency. In general, for a constant 
base line length, and constant precision of phase output and measurement, higher 
frequency means higher precision of a line of position and more ambiguities . Length- 
ening the base line and keeping modulation frequency and phase precision constant 
does not improve the line of position precision at points near the baseline, although 
as will be pointed out in the discussion of azimuthal systems the precision at points 
away from the base line is improved. This is simply saying that longer base lines 
give larger coverage areas for a given precision of result. The number of ambigui- 
ties is in general twice the number of wave lengths of the comparison frequency in 
the base line, the number of ambiguities being the number of lines of position along 
which the same phase or time difference will be measured. Another possibility is 
to transmit two different unmodulated radio frequencies from the two ends of the 
base line, and then to convert these to the same frequency at the craft for the phase 
comparison measurement. Here again the phase shifts introduced at the craft must 
be proportional to the frequency and constant in time. One might transmit unmodu- 
lated radio-frequency carriers at the same frequency for each antenna except that it 
would be impossible to distinguish the transmissions from the two antennas , and hence 
compare their phase. Since radio frequencies are much higher than the 833 cps used 
in the previous example the number of wave-lengths in the base line would be large 
as would the number of ambiguities in the pattern if the base length were kept the 
same. Furthermore the problem of maintaining radio frequency phase synchronism 
at the two ends of a long base line is difficult. Hence systems using phase compari- 
son at radio frequencies usually employ shorter base lines. A shorter base line 
implies a smaller region where the lines of position are approximately parallel. If 
the lines of position of any hyperbolic system are extended far enough out from the 
base they asymptotically approach straight lines radiating from the center of the base. 


c. Azimuthal or radial systems 

A hyperbolic system becomes an azimuthal system when the base line is a 
small fraction of the useful range of the system. At distances from the center of 
the base line greater than five times the length of the base the hyperbolae are essen- 
tially straight lines radiating out from the center of the base line. The hyperbola 
whichis the bisector of the base is exactly a straight line at all ranges. On the earth 
these radial lines of position are approximately great circles for all ranges greater 
than five times the base length. (The approximation is due to the possibility of mul- 


1.14 


Electronic Navigation Systems 



pattern. Spacing = X, transmissions in phase 


Electronic Navigation Systems 


1.15 


tiple transmission paths). As an example of an azimuthal system, suppose that the 
base line is 1.862 miles long and that the antennas at each end of the base line are 
driven in phase at a frequency of 100 kcps, the frequency whose corresponding wave 
length is 1.862 miles, so that the base line is one wave-length long. Figure 1-06 is 
simply an extension of Figure 1-05 which was for a case where the base was one 
wave-length long. The base line length is so small compared to the useful range 
that the points A and B are very close together and are not shown on the diagram 
although their line of centers is assumed horizontal as it was in Figure 1-05. The 
relative phase between the transmissions from the two antennas is given alongside 
the lines of position. Now however, the frequency is the same from each transmit- 
ting antenna and along the lines marked 180^ there will be phase cancellation so 
that noradiofrequency signals from A and B will be detected. Along the lines mark- 
ed 0® or 360° there will be phase addition and a maximum intensity of 100 kcps sig- 
nal will be received. The result of this is an interference pattern where the inten- 
sity of the received signal varies as the cosine of half the angle of phase difference. 
The dotted curved lines in Figure 1-06 are a polar plot of this function and are call- 
ed the horizontal pattern of the antenna arrangement of Figure 1-06. 

Figure 1-07 shows typical antenna patterns for various spacings and phase 
relationships of the A and B signals. In azimuth systems the lines of position define 
bearing angles on the base line of the system, so that the uncertainty of a line 
of position expressed in miles is directly proportional to the distance out from 
the center of the base line. The angular precision is highest near the bisector 
of the base and poorest along the base line extensions. If with a given system it is 
possible to control and measure phase to an uncertainty of ten degrees of electric 
phase angle then the geometric angular precision will depend on the number of 
wavelengths in the base line and on the angle between the particular line of position 
under consideration and the bisector of the base. Consider points near the bisector 
of the base line and assume the base line to be one-half wavelength long. The differ- 
ential phase angles are the same as those in Figure 1-04 and one has 30 electrical 
degrees compressed into less than 10 azimuthal degrees. So that if there were an 
uncertainty of ten electrical degrees, the geometrical line of position uncertainty 
would only be 3.33 degrees. If the base line is a whole wavelength then 60 electrical 
degrees correspond to less than 10 geometric degrees so that the same uncertainty 
of ten electrical degrees gives only 1.66 azimuthal degrees of uncertainty. As the 
base line is increased in length, the precision and also the number of ambiguities 
increase in direct proportion. In general, the maximum precision in a radial line 
of position attainable with a two- antenna azimuthal system near the base line bisec- 
tor is given by a simple equation. 

Uncertainty in miles _ Overall uncertainty in electrical degrees 

Distance to center of base in miles ” Number of electrical degrees in base 

. Thus if the system is capable of transmitting and measuring with an overall uncer- 
tainty of 3.6 degress and if the base is 360 degrees (one wavelength) long the uncer- 
tainty in distance will be one mile at 100 miles from the center of the base or 10 
miles at 1000 miles. If the uncertainty of electric phase angle can be held constant 
while raising the frequency and keeping the base length constant, or while lengthen- 
ing the base and holding the same frequency, the effect will be to increase the pre- 
cision in direct proportion. For a two-antenna azimuthal system the more general 
equation for any given line of position is given on page 1.22. This equation assumes 
that the base line length is much less than the distance from the observer to the 
center of the base, which is true for azimuthal systems by definition. The num- 
ber of ambiguities is twice the number of wavelengths in the base. 


1.16 


Electronic Navigation Systems 





Fig. 1-07 Horizontal radiation patterns, line of antennas horizontal 


Electronic Navigation Systems 


1.17 


All the discussion so far has been concerned with systems employing two 
antennas. Many azimuthal systems use more elaborate antenna arrays and variable 
phase control circuits to produce rotation or oscillation of antenna patterns. Others 
rotate the entire antenna array to produce the same effect. At the super high fre- 
quencies these antenna arrays take the form of reflectors and lenses having very 
sharp directional antenna patterns. With multiple antenna arrays the number of 
ambiguities increases, for a given spacing, but since the array intensity pattern be- 
comes more directional the side lobes are less pronounced and not as likely to give 
rise to ambiguities. Systems which can be rotated have the advantage that the only 
line of position actually intended for use is the base line bisector which is the most 
accurate line defined by the system. An example of a horizontal radiation pattern 
produced by three antennas is shown in Figure 17-03. 

Composite Systems 

A number of radar systems using such sharply defined antenna patterns 
yield both range and azimuth data and hence serve to locate the craft from one fix- 
ed installation. The range data define circular lines of position about the radar set 
and the azimuth data define radial lines of position which intersect the circles at 
right angles and give a "fix". The accuracy is constant for varying azimuth at a 
given range, in contrast with other systems in which the two position lines defining 
a fix do not always intersect at right- angles. Also the radial line of position is 
always the base bisector. 


II. Useful Range 

(a) Dependence on transmitter power 

Coverage implies that the navigation system is capable of maintaining con- 
tinuity in the supply of navigational information. Twenty-four-hour continuity is 
implied unless otherwise stated in a specific application. 

The signal strength necessary for satisfactory reception depends on local 
noise conditions at the receiver and on the noise generated within the receiver it- 
self as well as on the type of transmission and the method of presentation. Local 
noise varies with the geographical location, the season of the year, the time of day, 
and other factors many of which are unpredictable. Coverage areas often may be 
extended by increasing the transmitted power, but the increase of useful range at 
long ranges is very small even for large increases of power. For instance, referring 
to Figure 1-11, at 125 kcps, doubling the output power would extend the range for 
10 microvolts per meter signal strength of ground wave from 1530 to 1610 miles. 
And increasing the power by a factor of one hundred would only increase the 10- 
. microvolt ground-wave range to 2110 miles. The useful coverage area of a naviga- 
tion system is further determined by several factors. 

(b) Other considerations 

(1) The maximum range from fixed stations is governed by the same consi- 
derations that affect any radio transmission, fading and irregular or uncertain 
reception. 

(2) Interference between sky-wave and ground-wave transmission may pre- 
vent use of the system in certain parts of the coverage area and there may be 
skip regions. 

(3) The angle between lines of position may be so oblique as to make the 
precision of a fix too low. 

(4) The coverage area by day will in general be different from that at night. 


1.18 


Electronic Navigation Systems 


due to changes in the average noise level and to ionospheric conditions (see page 
1.26). 

(5) The useful range for aircraft using line of sight transmissions is extend- 
ed at high altitudes, and is subject to correction near the transmitting station, 
since range measurements are actually slant range and since the phase aspect 
(see Appendix A) of the transmitting towers changes rapidly as the elevation an- 
gle becomes large. Also atmospheric refraction affects maximum range slightly 

(6) Under certain conditions tropospheric effects, ’’ducting", etc. may great- 
ly affect the coverage area. 

Since in many cases coverage areas are limited by the precision attainable 
with a given system, most of the discussion of the above factors will be found under 
the heading of accuracy. 


III. Accuracy 

The accuracy of a "fix" depends on the precision of the line of position estab- 
lished by a navigation system and on the geometrical relationship of the two or 
more lines used to obtain the "fix". 

All electronic navigation systems depend on the measurement of a time 
interval, which in turn defines a distance (through the velocity of light). The spread 
of recent determinations of the velocity of light in a vacuum is shown in Figure 1-08. 
There is an uncertainty based on the above determinations of approximately 5 parts 
in 100,000 in the value of this quantity. The velocity of light in air is reduced from 
the value in a vacuum in the ratio 1/n, where n is the index of refraction of light in 
air, and is equal to 1.000294 for standard pressure and temperature conditions. For 
extreme ranges (p = 82 cm of Hg, t = -50°C; to p = 70 cm of Hg, t = +50°C) of tem- 
perature and pressure on the surface of the earth, the velocity of light could vary by 
as much as one part in ten thousand. Thus there may be a maximum uncertainty on 
the surface of the earth of 1.5 parts in 10,000; converted to actual distance this 
means, that without temperature and pressure corrections, there is an uncertainty 
of 80 feet in 100 miles or 800 feet in 1000 miles. It is evident that for most systems 
this is a negligible uncertainty compared to others which arise. Since atmospheric 
conditions may be quite accurately known, it is possible to correct for errors caused 
by them if extreme precision is necessary. The uncertainty could thus be reduced 
to approximately 0.5 parts in 10,000. The index of refraction at high altitudes 
approaches its free- space value which is unity. 

Since all navigation systems yield lines of position relative to fixed installa- 
tions, the geographical location of these fixed stations, beacons, reflectors, and 
natural reflecting or absorbing surfaces, must be accurately known. This consti- 
tutes a serious problem in parts of the world where surveys have not been accurate. 

The imcertainties in velocity of propagation, and in the location of geographical 
points on the earth, must be taken into account, but they are not peculiar to a parti- 
cular electronic system of navigation. 

There are three general categories of errors which depend on the design 
and operation of the navigational devices: 

(1) Errors arising in the measurement of time intervals. These intervals 
may be as long as several thousand microseconds or shorter than 1/1000 micro- 
second (10*9 seconds). These very short intervals are usually called phase 
differences. Errors in time or phase measurements may occur both at the fix- 
ed installations and in the navigated craft. 

(2) Errors caused by variations in the path taken by the transmission in get- 


Electronic Navigation Systems 


1.19 


o 

o 


o 


o 

OD 


o 


o 

CO 


o 

rr 



Fig. 1-08 Spread of recent determinations of the velocity of light in a vacuum 



1.20 


Electronic Navigation Systems 


ting from fixed station to craft, and vice versa. For ground- wave propagations 
the path is usually quite predictable, and the errors are smaller than those which 
arise in sky-wave propagations where the path includes one or more ionospheric 
reflections . 

(3) Errors due to ambiguities which the navigator fails to resolve. In many 
systems the navigator of the craft receives the same signal along more than one 
line of position and should be able to distinguish these by a fore-knowledge of 
his approximate position or by other means. 

The Time Measurement 

The generation of voltages defining standard time intervals, and the processes 
usedto compare such standards with the time intervals to be measured, are possible 
sources of error in all types of system. In a direct range, or circular system, the 
total time of travel of an electromagnetic disturbance is measured and converted 
to a distance by multiplying by the velocity of propagation. In differential range, or 
hyperbolic systems, the differential time is measured and converted to differential 
distance or directly to a line of position. In azimuth systems the differential time 
is in the form of an electrical phase angle which may be measured directly or ob- 
tained by measuring relative intensity of radiation patterns where the intensity is a 
function of transmitted radio-frequency phase, and also of the transmitter phase 
aspect as seen from the receiver. 

The measurement of an interval of time depends first, on the generation of a 
standard frequency which furnishes a continuous series of equal time intervals 
(cycles) and is the time “measuring stick"; and second, on some means of compari- 
son so that the time interval to be measured may be compared with a number of 
cycles or a fraction of a cycle of the controlled frequency. The precision of crystal- 
controlled oscillators is easily of the order of one part in a million, and may be as 
good as one part in 10^ if extreme care is taken with temperature control and so 
forth. For direct range measurement one part in a million corresponds to a preci- 
sion of 0.528 feet in 100 miles which is a negligible error. High-“Q" ringing circuits 
and other timing devices may be used where a lower order of accuracy is sufficient. 
Unless very long time intervals must be measured, the precision of a crystal-con- 
trolled frequency is likely to be much better than that of the comparison operation. 
The problem is not unlike the measurement of time intervals with a clock. Assume 
that the clock runs at a uniform and accurate rate: this is equivalent to saying above 
that the oscillator operates at a constant frequency. If the clock has a second hand, 
which revolves at one revolution per minute, one could measure an interval of one 
hour to an approximate precision of 1 part in 3600 by measuring time intervals to 
the nearest second. To measure the same time to a higher precision one would have 
to divide the distance moved by the second hand in one second into smaller intervals. 
Since the number of angular subdivisions on the face would have to be increased, it 
would be necessary to use a narrower pointer or one whose alignment with fixed 
marks on the clock face is accurately discernible. This requires both a narrow or 
sharp edged pointer and sharply defined marks on the face. It may be necessary to 
mount a hand on the clock which moves around ten times a minute or even faster, in 
order to spread out the time scale. The various electronic timing and indicating 
circuits make possible a precise measurement of much shorter time intervals than 
ordinary clocks but have the same basic limitations. The electronic measurement 
of phase differences is a typical time measurement. In making a phase comparison 
it is easily possible to measure relative phase with uncertainties of the order of 
five or ten degrees. It is much more difficult to measure phase angles with maxi- 
mum uncertainties of the order of one half degree. One one-hundredth of a cycle, or 
3.6 degrees, represents a typical uncertainty for phase angle measurements. At 
100 cps, 3.6 degrees of phase angle corresponds to 100 microseconds, so that with 
such a coarse “measuring stick" it would be extremely difficult to measure times 
with uncertainties of the order of one microsecond. Using a comparison of 100-cps 


(This page is inserted as the simplest means of correcting the inadvertent omission 

of Figure 1-09.) 


1.20a 


Electronic Navigation Systems 




Fig. 1-09 Comparison of continuous lOkcps modulation envelope with a single pulse 
shaped like one full cycle of the modulation envelope as shown. Periodograms show- 
ing the spectral distribution for each type of transmission are shown on the right. 
(Note that the 10 kcps side band amplitudes for the continuous wave case have the 
same value as the distribution curve for the pulse at carrier frequency plus or minus 

10 kcps.) 


Electronic Navigation Systems 


1.21 


modulation envelopes for a system such as that discussed on page 1.10, one could 
measure times up to 10,000 microseconds (assuming that the transmissions from 
thetwostations are distinguishable), corresponding to base-line lengths of the order 
of 1,000 miles, with an uncertainty of the order of 100 microseconds, and have no 
ambiguities, li higher accuracy were necessary it would be better to use a 1,000- 
cps modulating frequency for the phase comparison. This would give uncertainties 
oHheorder of 10 microseconds for the same precision of phase angle measurement, 
but would give rise to 10 ambiguities or 10 lines of position along which the same 
phase difference is measured. In order to reduce uncertainties to values of the 
order of 1 microsecond it would be desirable to use a modulating frequency of the 
order of 10,000 cps, which would in turn give 100 ambiguities. An expedient for 
resolving the ambiguity in this case is to omit 99 out of every 100 cycles, transmit- 
ting a single complete 10, 000- cps cycle every 10,000 microseconds. This cycle be- 
gins and ends at the zeros of output voltage in the modulated output assuming 100% 
modulation. The equation for the amplitude (e) of the upper envelope of such a pulse 

is e = "I" + " 2 ^®® 27rlO^. The modulation envelope becomes in this case a pulse 

having a length of 50 microseconds at the half -amplitude value. Fig. 1-09 shows such 
a pulse for comparison with the 10-kcps modulation. On the right are shown the 
spectral distributions of transmitted frequencies for the two types of transmission. 
It is evident that receiver band width requirements will be similar for the two 
transmissions; this point is further discussed under frequency and bandwidth. 

Phase comparisons are simplest and usually most accurate when the compari- 
son is between waveforms having 0^ or 180^ phase difference. Many direction hnd- 
ers can detect phase deviations as small as one- half a degree from zero. In this 
case the phases of a wave arriving from a distant transmitter at two parts of a 
receiving antenna array are compared and the array is turned physically until the 
phase difference is zero, indicating that the array is parallel to the arriving wave- 
front. The final measurement is always made at zero phase difference. In other 
systems, a voltage corresponding to one of the two waveforms whose phase differ- 
ence is to be measured, is the input to a calibrated, continuously-adjustable phase 
shifter, which is adjusted for zero phase- difference between its output and a voltage 
corresponding to the other wave. The comparison is thus made at zero phase differ- 
ence and the actual phase difference is read off the dial of the phase shifter. This 
procedure adds the possible errors in the phase shifter to those of the recognition 
of zero difference. Certain other types of phase measuring devices have larger 
errors near zero difference than elsewhere. Systems which define lines of position 
as lines of constant radio-frequency phase difference must transmit accurately 
phase- controlled carriers from each transmitting antenna, and at the receiver the 
circuits must not introduce different phase shifts into the two transmissions . Further- 
more, it is necessary to identify transmissions from the different antennas. This is 
done in the POPI system by a keying sequence, transmissions from different antennas 
occurring successively at a single frequency. In this case the receiver circuits will 
not give rise to differential phase shifts. However, it is necessary to have a phase 
"memory" circuit in order to compare phases of voltages produced at the reception 
point by the different transmitting antennas, since they do not occur simultaneously. 
The Decca system transmits simultaneously at different frequencies from the two 
or more antennas of the fixed station. These carriers are converted to a common 
frequency at the craft and compared. Assuming accurate phase control at fixed sta- 
tions, the phase shifts in the receiver' s radio frequency and converter stages must 
be accurately controlled, since the two transmissions come through different ampli- 
fier channels. This requirement in turn precludes the very sharply tuned circuits 
which might at first seem desirable for continuous-wave reception and noise exclu- 
sion. Any variation in the time delay between transmitted pulses in a pulse system, 
or any shift in phase of a modulation envelope, or radio frequency carrier, will 


1.22 


Electronic Navigation Systems 


shift the whole pattern of lines of position, and hence must be accurately controlled. 
Many azimuthal systems define lines of position which are rotated either by rotat- 
ing the entire antenna assembly, as in various radar systems, or by systematically 
and continuously shifting the phase of different antennas of the transmitting array, 
as in the Sonne system or the various omni-" ranges". In either case the phase 
control is a possible source of error in the resultant lines of position. In certain 
azimuth systems the time measurement, or rather time definition, is entirely done 
at the transmitter. 

A system of the AN type of radio beacon defines a line of position as a line 
along which two overlapping patterns have the same intensity. These patterns might 
be produced by keying antenna currents to produce the two patterns shown in Fig. 
1-07 (a) and (b). The pattern of Fig. 1-07 (a) is keyed as an A (— ) and the pattern 
of Fig. 1-07 (b) is keyed as an N (-•) which interlocks in time with the A to pro- 
duce a continuous tone along the lines where the two patterns have the same inten- 
sity, which is known as the equisignal, Electra operates on a similar principle ex- 
cept that the keying of the patterns is by interlocking dots and dashes, and the pat- 
tern used is multilobed, giving higher precision but more ambiguities. These sys- 
tems only define exact position along a finite number of lines. Sonne rotates the 
Electra pattern and thus enables the navigator to find his line of position over the 
entire coverage area. In the Federal long-range system, lines of position are de- 
fined by relative intensity measurements on four overlapping patterns . All these 
systems depend on accurately phased antenna currents, which constitute a timing 
problem. The numerical relationship between phase angle uncertainty and azimuth 
angle uncertainty is given by 

A0 = - ^ 

277n cos 0 R . 

Where A0 is the uncertainty in azimuth angle 

A<^ is the overall uncertainty of electric phase angle 
n is the number of wavelen^hs in the base 

0 is the azimuth angle measured from the perpendicular bisector of the base 
AS is the lateral uncertainty in line of position in miles 
R is the distance to the center of the base in miles. 

A numerical example for the Sonne system is worked out on page 17.24. 

Whether the time comparison is between radio frequency cycles, or sinu- 
soidal or pulse modulation envelopes, the real criterion for time uncertainty is the 
steepness of slope of the wave form at the point of comparison. The rise time for 
a pulse is usually taken as the time required to get from 10 to 90 percent of the 
peak pulse amplitude. For a pulse form like that of Fig. 1-09, this time corresponds 
to 108 degrees of phase angle or about thirty times the typical maximum phase un- 
certainty of 3.6 degrees used above. For pulses, a rise time of 25 or 30 times the 
maximum allowable uncertainty is a reasonable engineering choice. Since rise time 
is inversely proportional to band width of transmitted pulse waveforms, one attempts 
to keep rise times as long as possible. This question is discussed further under 
frequency and bandwidth. The relation of rise time for pulses to maximum time un- 
certainty also depends on the type of presentation. 

If the pulses are being displayed and compared on a PPI scope, where the 
pulse modulates the beam intensity, it is necessary to have the duration of the pulse 
of the same order of magnitude as, or somewhat shorter than, the maximum tolerable 
uncertainty. Thus to measure times with an uncertainty not greater than one micro- 
second, the pulse rise time should be somewhat less than a microsecond. On the 
other hand where the pulse is displayed as a vertical displacement (as in Loran) or 
horizontal or radial displacement (as in Shoran) against time as the other coordinate, 
the pulse may be of considerably longer duration than the maximum tolerable uncer- 


Electronic Navigation Systems 


1.23 


tainty. In these cases the relative sizes of CRO spot, and the total spread of the 
pulse determine the minimum uncertainty. With the latter presentation, it is usual- 
ly desirable to have the entire pulse on the screen for monitoring of amplitude so 
that pulses of similar forms may be compared. Thus if the total trace len^h is 200 
microseconds (as in Loran) it is quite possible to make pulse comparisons with im- 
certainties of the order of one microsecond. In all cases small, sharply focused 
spots are necessary for such measurements and the noise level must not he too high. 
In Loran or Shoran noise tends to make a multiple or broadened trace which is not 
capable of fine resolution. In PPI presentations noise clutters the screen and tends 
to reduce the sharpness of the picture. No general statement can be made as to the 
effects of noise on oscilloscopic presentations since noise wave forms may have any 
of an infinite number of possible shapes. No noise is assumed here in the horizontal 
or time base signal. If the pulse is modulating a low frequency carrier whose period 
is a substantial fraction of the rise time then the position in time of the front edge 
will flutter by the amount of the carrier period unless the phase of the carrier bears 
a constant time relationship to the pulse envelope. 

The Propagation Path 

The second general class of errors in navigational systems are those aris- 
ing from deviations from the expected transmission path or paths. Radio transmis- 
sions in free space travel straight outward from a source, like other electromagne- 
tic radiations. Radiations starting from sources near the surface of the earth will 
encounter various obstacles, and will travel through the earth's atmosphere. The 
radiations may be reflected or absorbed by obstacles. In general, diffraction will 
occur, so that obstacles of small dimensions relative to the wave length will not cast 
clearly defined radio shadows. Since the atmospheric density decreases with height 
there will be refraction, tending to bend the radiation path towards the earth. Radia- 
tions originating near the surface of the earth tend to bend around the curvature of 
the earth by a combination of refraction and diffraction. Since the earth is an im- 
perfect conductor there will be ohmic losses associated with the passage of an electro- 
magnetic wave over the earth' s surface. These losses will tend to attenuate the 
radiation intensity near the surface. If the earth were a flat, perfect conductor the 
radiationwoulddecrease in intensity inversely as the distance from the source. This 
is practically the case for transmissions well above the surface of the earth, as in 
line of sight transmission from plane to plane. Actually the attenuation along the 
surface is greater than that predicted by the inverse first power of distance due both 
to the curvature and the poor conductivity. Norton' s Formula^ takes account of these 
factors and its reliability has been experimentally verified down to a frequency of 
ISOkcps, and there is no reason to doubt it at frequencies below this value. Figures 
1-10 and 1-11 are graphs of Norton's formula for a range of low and medium fre- 
quencies, transmitter and receiver being at the earth's surface and transmission 
occurring over seawater. The inverse first power of distance law is plotted on each 
graph for reference. At high frequencies ground losses are high and diffraction 
bending is less pronounced. This direct transmission is known as gro\md-wave pro- 
pagation. Under certain conditions, the expected ground- wave range may be greatly 
increased by "ducting" or "trapping" of a wave between the earth' s surface and in- 
version layers in the atmosphere above the earth. This phenomenon is particular- 
ly common over sea water. 

The other important radio transmissions between points on the earth's sur- 
face are known as sky- waves, since they are propagated by means of reflections from 
ionized layers of gas in the upper atmosphere. The free electrons in the rarefied 
gases of the outer atmosphere behave like any free electrons in that they move in re- 
sponse to electromagnetic radiation. The motions of free electrons in a metal are re- 


* K. A. Norton, "The Calculation of Ground- Wave Field Intensity Over a Finitely 
Conducting Spherical Earth", Proceedings of the I.R.E., December, 1941. 


1.24 


Electronic Navigation Systems 



Fig. 1-10 Graph of Norton' s formula for transmission over sea water, both scales 

logarithmic 

sponsible for its ref lecting properties . Free electrons in the ionosphere will reflect 
impinging radiation if the electron density is sufficient. The motions of the free 
electrons may be hampered by collisions with gas molecules. This process will 
involve loss of energy and therefore absorption of energy from the radiation which 
originally caused the motion. The ionizing of layers of gas is caused mainly by ul- 
traviolet, and partly by corpuscular radiations from the sun, so that the electron 
density in a given layer is governed by a balance between the arrival of ionizing 
energy from the sun and the continual loss of free electrons due to attachment to 
atoms and molecules, and to recombination with positive ions. Since the ultra-violet 
and corpuscular radiations from the sun may fluctuate greatly in intensity, the re- 


10.000 


Electronic Navigation Systems 


1.25 



d3d SllOAOdOII^ 


400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 

STATUTE MILES OVER SEA WATER 6-80 (T- 5 X lO"" 

Fig. 1-11 Graph of Norton' s formula as in Fig. 1-10 but with linear distance scale 


1.26 


Electronic Navigation Systems 


suiting density of ionization will also fluctuate. The ionized layers are diffuse in 
structure and hence do not present a sharply- defined reflecting layer. For this 
reason the apparent height of the reflecting layer depends on the angle on incidence 
of the radiation as well as on the frequency. For vertical incidence the apparent 
height of the reflecting layer is greater than at grazing incidence. At grazing inci- 
dence the radiation travels for a longer distance through the lower fringes of the 
layer and is therefore subject to greater absorption losses due to collisions. The 
lower important layer is known as the "E" layer, which has an approximate effective 
heightof seventy miles for a vertical incidence reflection. The upper layer is known 
as the “F” layer and is approximately 190 miles high. The "F” layer is a relative- 
ly thick layer, of greater electron density, having boundaries in which this density 
tapers off gradually. It often exhibits two maxima of electron density at different 
levels. The ”E" is at present the more important layerfor systems using pulse 
transmissions. For grazing incidence the effective height is lower, being approxi- 
mately fifty-four miles for two- megacycle transmissions leaving the earth tangent 
to the earth* s surface. There is evidence that this effective height for grazing in- 
cidence reflection is lower than fifty-four miles at frequencies below two megacycles. 
At the present time, data on ionospheric reflections for other than vertical incidence 
are systematically predicted from measured vertical- incidence data. However, there 
are practically no ionospheric data of any sort at low frequencies . Although the ultra- 
violet radiation from the sun is cut off at night, the recombination of molecular oxy- 
gen takes place slowly and a sufficient electron density persists through the night 
to give good reflection at the lower frequencies. In fact, although the "E" layer is 
denser in daytime than at night, it also extends to lower levels where the atmospher- 
ic gases are more dense, and hence where energy absorbing collisions are more 
likely to occur. At night, the "E" layer is more tenuous but also more sharply de- 
fined, and ref lected radiations are subject to less attenuation due to passage through 
regions where the collision rate is high. Since the higher-frequency radiations re- 
quire a greater electron density for reflection, there are practical limitations to 
the usefulness of sky- waves at various frequencies. At frequencies above 60 mcps 
ionospheric reflections are very rare. Between 60 and 30 mcps reflections may 
occur but are not reliably predictable, and hence not useful for transmission pur- 
poses although they may give rise to undesirable interference. Below 30 mcps fair- 
ly reliable predictions are available. In general, at frequencies below 20 mcps 
transmission from one point to another on the surface of the earth will be both by 
ground- wave and by one or more sky-wave types of propagation. (Sky- waves embrace 
one- hop or multiple- hop "E"-layer paths and one or more "F"-layer transmissions.) 

Variations in lines of position in navigation systems which are directly or 
indirectly caused by sky-wave transmissions fall into two categories. First, those 
which arise from the fact that sky- waves exist as a mode of propagation in addition 
to ground waves. The secondsort of errors are those arising from abnormal varia- 
tions in the behavior of the reflecting ionospheric layers. Cpnsider the first type 
of error. The sky-wave path between two points on the surface of the earth will 
always be longer than the ground- wave path. Speaking in terms of time, for points 
near the transmitter the one hop "E "-layer sky-wave will require approximately 
600 microseconds longer time for transmission than the ground wave. As the re- 
ceiving point is moved away from the transmitter, the amoimt by which the sky-wave 
path is longer than the ground- wave path approaches a nearly constant value which, 
in terms of time, is approximately sixty-five microseconds at extreme one- hop ranges 
for two-megacycle transmissions. This time difference between sky-wave and 
ground- wave transmissions, having common transmitting and receiving points, is 
known as the sky-wave delay- time and has important bearing on navigation- system 
design. Since this delay time decreases with increasing distance, the phase of a radio 
wave coming by sky-wave propagation will differ from that of the ground wave by a 
variable amount depending on the distance traversed. And furthermore, since the 


Electronic Navigation Systems 


1.27 


received radiation is the instantaneous sum of ground- wave and one or more sky- 
wave components, its resultant phase and amplitude may vary with distance quite 
differently from the phase and amplitude variations of the ground wave alone. 

A second source of error in systems using spaced antenna arrays may be 
present even with perfectly normal sky-wave reflections. Sky-wave transmissions 
may leave an antenna array at substantial elevation angles. When this occurs, the 
phase aspect of the transmitting antennas is different for sky-wave and ground- wave 
propagation. This is equivalent to saying that the differential distance to two anten- 
nas by the sky-wave route is in general different from the differential distance by 
the ground- wave path. The result is that in hyperbolic and azimuthal systems, the 
lines of position defined by the system are different for sky- and ground- wave propa- 
gations. Figure 1-12 shows in solid lines a typical set of ground- wave position lines 
for an azimuthal system and in dotted lines the pure one-hop sky-wave position lines 
for the same phasing of the antenna currents and the same antenna array. The dotted 
lines in this case are calculated for a uniform ionospheric height of fifty-four miles. 
A similar situation exists for hyperbolic lines of position. Note that the effect is 
not present along the base-line bisector (vertical line in Figure 1-12) and is largest 
as one approaches the base-line extension (horizontal line in Figure 1-12). The dot- 
ted lines converge toward the solid lines as the range increases, since the one- hop 
sky-wave leaves the antenna array at nearly zero elevation angle for extreme ranges, 
so that the transmitting array has the same phase aspect as for the ground wave, 
and therefore produces the same radiation pattern. However, two- hop "E "-layer 
and "F "-layer transmissions may predominate, and in this case the sky-wave cor- 
rection may have to be applied even at extreme ranges. All the above effects can be 
compensated for if it is possible to separate sky-wave from ground- wave propaga- 
tion, and if one has available a reasonably good prediction of seasonal and diurnal 
variations in the effective height of the ionosphere or expected sky-wave delay time. 
There are several possible methods of separating ground waves from sky waves. 
First, by being so near the transmitter that ground waves are strongly predominant, 
or far enough away that ground waves are reduced to negligible relative value. (There 
is in this latter case the strong possibility that the sky-wave may consist of both 
one-hop and multiple-hop transmissions which may be inseparable.) Second, by con- 
trolling the elevation angle of transmitted and received radiations so as to utilize only 
one path at a time. This is only practical at the higher frequencies, where sharply 
directional antenna systems are feasible. Third, by transmitting intermittently so 
that the firs^t radiation received after a period of no transmission is that which trav- 
eled by the shortest route, which is the ground wave. The various pulse- modulated 
systems make use of this method of separation of ground and sky waves. If the pulse 
is substantially shorter than the minimum delay time of sixty-five microseconds, 
then ground- wave and sky-wave pulses will be distinguishable at all ranges, and 
either may be used for measurement purposes. Fourth, by transmitting at higher 
frequencies, so that ionospheric reflection does not occur. 

With pure ground waves there is no need to be able to predict sky-wave pro- 
pagation conditions, but in order to use sky waves it is necessary to know what to 
expect for delay times and usable frequencies . 

Since sky waves must travel a distance at least twice the effective height of 
the ionosphere, roughly one hundred and ten milesfor 2-mcps transmissions, the 
sky-wave intensity will normally be a small fraction of the ground- wave intensity at 
points near the transmitter; but, at ranges of the order of one hundred miles and 
more, the sky wave may be much stronger than the ground wave, especially over 
land. This is particularly noticeable at frequencies from ten to thirty megacycles 
where the ground- wave attenuation is high and where ionospheric reflection may be 
good. As the frequency is lowered, the distance out to points where ground- and 
sky-wave field strengths are comparable, increases. With present available data it 


MILES 


1.28 


Electronic Navigation Systems 



Fig. 1-12 Relative location of azimuthal lines of position for ground wave (solid 
lines) and one- hop E- layer sky wave (dashed lines). Base line horizontal, 
first quadrant only shown 

Note: lines of position determined by two- hop E-layer sky wave and one- 
hop F- layer sky wave will deviate from ground- wave lines by larger amounts 



Electronic Navigation Systems 


1.29 


is not possible to give numerical figures in this connection. Such data would be very 
useful in evaluating the potentialities of low frequencies for navigation systems. 
Several things need to be ^own in this connection for very low and low frequencies. 
First, what will be the sky-wave delay time for oblique- incidence ionospheric reflec- 
tions. Second, how serious will sky-wave attenuation be, and will two-hop trans- 
missions be of comparable field strength to one-hop transmission, since the one- 
hop propagation has a longer path through the lower attenuating fringe of the "E" 
layer. For a given range, an ''F"-layer or a two-hop ''E"-layer transmission takes 
off from the transmitting array at steeper elevation angles than for the one-hop "E”- 
layer path. 

Systems using sky-wave transmission use a chart designed for ground- wave 
transmission by applying corrections based on a certain expected delay of sky waves. 
The correction may be equivalent to as much as 50-60 microseconds. Whether or 
not sky-wave corrections are necessary depends on whether the ground wave recep- 
tion zone extends out to regions where the single-hop ”E" -layer reflection paths 
leave the transmitter system at low elevation angles. Since the effect depends on 
the cosine of the elevation angle of the propagation path from the horizontal, eleva- 
tion angles less than eight degrees will give rise to corrections which are less than 
1% of the phase aspect at the point in question. If one assumes an ''E'‘-layer 60 miles 
high, the ground- wave range would have to be approximately 700 miles in order to 
avoid appreciable corrections. This fact in turn will have a bearing on the choice 
of frequencies for systems depending on both sky- and ground- wave propagations. 
In general it is not desirable to use ground- wave propagation from one station of a 
transmitting pair and sky- waves from the other. This procedure is not at all im- 
possible, but since sky-wave corrections would involve total sky-wave delay time 
and not the difference of two such times, the precision would be lower. 

All the possible sources of error so far discussed arose out of a considera- 
tion of normal predictable ionospheric conditions, the uncertainties being as to 
which paths the radiation actually followed. There are also the types of errors 
which are due to unpredictable fluctuations. These fluctuations in sky-wave field 
strength are probably due to motions and density variations of the electrons in the 
reflecting layer. These motions give rise to a variation in the sky-wave delay time 
which has an experimental value ranging up to more than forty microseconds, and 
has a twenty- minute average which may vary by twenty microseconds from one 
twenty- minute interval to the next. Variations in sky-wave delay time may be 
thought of as variations in the effective height of the ionosphere. Because of this 
variation in delay time, any use of sky- waves for direct- range systems would 
involve rather large uncertainties. The same consideration has a bearing on the 
possibility of maintaining synchronism between a pair of hyperbolic- system trans- 
mitters. If it is possible to separate sky-wave from groimd-wave transmission, it 
is possible to maintain synchronism with ground- wave signals, with corresponding- 
ly good precision, up to the limit set by the ground-wave intensity and local noise 
conditions. It is further possible to synchronize by single-hop "E "-layer sky- waves 
provided that there are means of separating this sky-wave component, and of main- 
taining watch to effect an intelligent smoothing of the fluctuations, and provided that 
a lower order of accuracy can be tolerated. In differential- range and azimuth 
systems, the difference between two sky-wave arrival times is the measured quan- 
tity, and if the variations in delay time are the same for each sky-wave then they 
will cancel out in the difference. This will be the case at points along the base line 
bisector where the distance to each transmitting antenna is the same (assuming 
changes in the effective height of the reflecting layer to be identical all over the 
coverage area). For points near the base line extension, the difference in ground- 
wave distance to the transmitting antennas has a maximum value, and even though 
the effective ionospheric height is the same at each reflecting point the variations in 
this height give rise to variations in the measured difference between times of 


1.30 


Electronic Navigation Systems 


arrival of sky- waves . For azimuthal systems which have relatively short base lines 
(two or three miles), the reflecting points are near together and hence more likely 
to be at the same effective height, and to vary in effective height together. On the 
other hand with long base lines it is much less likely that the effective ionospheric 
height is the same at both reflecting points or that it will vary in the same way at 
the same time. Thus with continuous -wave systems using sky- waves or composite 
transmissions, the shorter base lines are better from the point of view of propaga- 
tion uncertainties. Ionospheric tilt or ‘'patchiness" may interfere seriously with the 
precision of a line of position from azimuth systems and its effect may be to dis- 
place any of the lines of position of the system. Tilt is equivalent to horizontal 
electron-density gradient. 

Pulse- modulated systems offer the possibility of separating ground wave 
from sky wave and hence of making measurements on one component of the signal 
at a time. For instance, suppose that transmitted pulses have a duration substan- 
tially shorter than sixty-five microseconds. Then the ground- wave pulse would 
always be completely over before the same transmitted pulse travelling by the sky- 
wave route arrived at the receiver. There would be no interference and one could 
measure difference in times of arrival of ground waves or sky waves independent- 
ly. However, as will be pointed out in the discussion of frequency and bandwidth, 
the length of a pulse is not a quantity which may be chosen at random for any fre- 
quency of transmission. As the frequency is reduced, the difficulties in the way of 
producing and using short pulses increase. This usually means longer pulses and 
hence overlapping of sky wave and ground wave. The technique of cycle- matching 
(suggested for LF Loran) on the early parts of two pulses whose time-difference is 
to be measured has some attractive possibilities. 

The leading edge of a received pulse is unique in that for the first sixty-odd 
microseconds of its build-up time it will consist of pure ground- wave transmission, 
uncontaminated by sky- waves, provided that the receiver is within ground- wave 
range. If one could perform a phase match between radio-frequency cycles of two 
pulses during this early part of the pulse it would be possible to achieve the high 
time-precis ion which is obtainable with a phase measurement at the radio-frequency. 
This possibility hinges on certain identification of the first few cycles so that the 
right pair are matched. This identification in turn requires a rapid rise of the front 
edge of the pulse and therefore relatively wide band- width allotments. 

Errors due to ambiguities 

Two circles or two hyperbolae may intersect at two points. Either of these 
two points may be the actual position of the craft. Unless some third line of posi- 
tion exists, or unless the navigator knows his approximate position by other means, 
there is an ambiguity of position. 

Azimuth systems having multilobed antenna patterns have sector ambiguity. 
That is, the signal received from the system is the same along several radial or 
hyperbolic lines. There are two factors which influence original design in these 
cases. If a system has few lobes, the number of sector ambiguities is less but the 
precisionwithin a sector is also reduced for the same phase or amplitude discrimi- 
nation. Many multilobe systems use a large number of lobes for high angular pre- 
cision, and require an addition direction-finding equipment to locate which sector 
the craft is in, or a second "coarse" radiation pattern for the same purpose. It 
is essential that if the navigation system has ambiguities there be some means 
of resolving them. Furthermore, the sectors of ambiguity must be sufficiently 
broad that the craft cannot move across a sector in less than two or three times the 
time required to obtain a fix. If for instance a system has a ten-degree sector, 
then at a distance of 100 miles from the antenna array, the sector is approximate- 
ly seventeen and one half miles across. Flying at 300 miles per hour at right 


Electronic Navigation Systems 


1.31 



a 

0) 

09 

CO 

0) 

I 

0) 

S' 

u 


a 

•fH 

t 

3 

a> 

a 


CO 

Pi 

< 

CO 


bn 

pp 


1.32 


Electronic Navigation Systems 



Fig. 1-14 Uncertainty in line of position, hyperbolic system 


Electronic Navigation Systems 


1.33 


angles to the radial lines, a plane would cross the sector in three and one-half min- 
utes. If the time required to obtain a fix is of the order of two minutes, it is obvious 
that the system would not be useful at ranges so close to the transmitter. It is evi- 
dent that for regions sufficiently distant from the transmitting station, sector ambi- 
guities fall into the category of gross errors which would only be a serious problem 
for craft which are completely lost. 

Geometrical considerations in the use of two or more lines of position for a fix 

Since time measurements and propagation path predictions are subject to 

error, a "line" of position established by the system has a degree of uncertainty. 
Suppose the navigator knows his exact position by other means, then a series of 
measurements using a navigation system which has no systematic errors will 
yield readings which on the average will form a statistical ensemble about the true 
value. It is then possible to draw two lines of position which form boundaries to a 
region within which 907* of a larg^ number of position-line measurements will fall. 
These lines will straddle the true line of position. If the navigator wishes to deter- 
mine his position, assuming that he does not know where he is, he can take a 
measurement and know that there are nine chances out of ten that his true position 
line is within a similar zone about his measured value. The geometrical shape of 
such a zone depends on the type of system and on the timing and path uncertainty. 
Figure 1-1 3 shows such zones for two circular systems. When a "fix" is taken using 
circular position "zones" from two stations such as A and B of Figure 1-13, there 
is an 8.1 out of 10 chance that the true position of the craft lies in the quadrangular 
area a, b, c, d. The quadrangular area e, f, g, h, in Figure 1-13 represents similar 
"zones" intersecting at a less favorable angle; there is still an 8.1 out of 10 chance 
of including the true position in the area but the area is larger and the true position 
therefore less accurately known. Figure 1-14 shows the shape of the zone of uncer- 
tainty about a hyperbolic line of position. Note that in Figures 1-13 and 1-14 the 
assumption is made that the measure of distance or time has the same uncertainty 
regardless of total distance or time measured. This will be true for all errors due 
to incorrect phase comparison or pulse alignment, but will not be true for errors 
arising from frequency drift or variations in either the velocity or the path of pro- 
pagation. In Figure 1-14, the zone of uncertainty approaches a sector of constant 
angular width as the hyperbolae approach radial asymptotes. Figures 1-15 and 1-16 
show similar constructions for zones of uncertainty in azimuthal systems. It is 
assumed that 907o of bearing measurements will fall within 1^ of the true value. 
Then at a distance of 100 miles from station A there are nine out of ten chances that 
the true line of position lies within 1.75 miles of the measured value, 0.0175 being 
the tangent of 1®. If then two measured lines of position intersect at an angle near 
90® at a point not more than 100 miles from either station, the position of the craft 
is somewhere in the diamond shaped area shown shaded in Figure 1-15. In this case, 
the maximum probable distance between the actual position of the craft and its cal- 
culated position is of the order of two and one-half miles for the case assumed. If 
the angle of intersection of the two lines of position is for example 15 degrees, the 
result is shown in Figure 1-16, where the maximum probable error is approximately 
16 miles at 100 miles from the fixed station. If one plots the region within which the 
angle of intersection of two lines of position is equal to or greater than any given 
value, the boimdary of that region is found to be a circle passing through the two 
fixed stations. This is illustrated in Figure 1-17, where the circle passing through 
the two stations encloses a region where the angle of intersection of azimuthal lines 
of position is always equal to or greater than 30®. At any point on such a circle, the 
angle of intersection of azimuthal lines of position is the same. At any point on a 
similar circle dr awn for two stations constituting a hyperbolic system pair, the zone 
of uncertainty will be approximately the same width measured perpendicular to the 
hyperbolae as in Figure 1-14 where ab = cd = ef. Such a circle may be used to de- 
lineate regions of coverage having a precision equal to or better than a given value. 
If in range systems it is also possible to measure differential range, then the area 


1.34 


Electronic Navigation Systems 



Fig. 1-15 Uncertainty in a fix determined from azimuthal lines of position, angle 

of cut near 90® 

of uncertainty may be reduced by a substantial factor as shown in Figure 1-18. The 
diamond shaped area in black dashed lines is reduced to a hexagonal region between 
the dotted lines. In this case it is assumed that the precision of the differential 
range measurement is the same as that of the direct range measurement. 

IV. Type of Presentation 

The presentation refers to the manner in which the navigator is made aware 
of the navigational information. In certain cases it may be desirable to by-pass the 
navigator or pilot and feed the course data directly to the steering control mechan- 
ism. Aside from this "automatic" operation, the navigational data reaches the con- 


Electronic Navigation Systems 


1.35 



Fig. 1-16 Same as Fig. 1-15, but with angle of cut 15^ 


1.36 


Electronic Navigation Systems 



Fig. 1-17 Boundary of a region within which the angle of cut is equal to or greater 

than a given value 

sciousness of the navigator by either visual or aural means. It is in general true 
that it is possible to present more information in a given time by visual means than 
by aural means. However, a glance at the various instruments visible from the 
pilot' s position on an aircraft, and the recognition that he has to see objects out- 
side the plane also, will indicate that in most cases of aircraft navigation, the pilot's 
visual channels for the reception of information are pretty well saturated. Whatever 
reaches the pilot by aural means arrives by way of a set of earphones which, in some 
cases, may be switched to several circuits (interphone, various radio communication 
channels, etc.). 

For single-place fighter craft it is desirable to have navigational informa- 
tion presented in as simple a manner as possible. Homing information or prese- 
lected course flying lends itself easily to simple presentations such as right-left 
visual or aural indications. In this case accuracy is subordinate to simplicity. For 
aircraft direction or "vectoring" from ground stations, such as in radar fighter-dir- 



Fig. 1-18 Illustrating the reduction in uncertainty of position by combination of 
circular and hyperbolic lines of position 


ection, the presentation is via voice communication channels from the control point, 
and high precision is possible since it is determined by the ground installations. A 
similar system is useful for airport traffic control. For general navigation of fight- 
er craft the problem is more difficult. Suppose a fighter craft after a several min- 
utes "dog fight" or evasive action is completely lost. Assuming that he has a com- 
pass, he then needs to be able to obtain a "fix" in order to proceed with his assign- 
ment. This implies that he must be able to take some sort of a chart of the region 
where he is and put a dot, labeled with a time, on it to represent his position. It 
may also be desirable to plot a series of fixes thus defining his course. While the 
presentation of general navigational data in a useful form to a fighter pilot is a very 
serious problem, it is relatively much easier to do for the navigators of large air- 
craft and ships since in these cases the navigator usually does not have extensive 
other activities. 

Aural Indication 

Under aural types of presentation there are three useful subdivisions: 

(a) Direct verbal or code instruction via radio communication circuits from fixed 
stations or other craft. This is sometimes called "vectoring", 

(b) Homing or other preselected course navigation by "dot-dash" or A-N direct 
error indication. 

(c) General navigation by aural recognition of some form of identification of lines 
of position. 

Direct verbal instructions are used in radar fighter or bomber control and 
in airport control systems. The ground or ship radar controller "sees" the craft 


1.38 


Electronic Navigation Systems 


in its relation to other craft and fixed objects , and directs the navigation. This implies 
that some means of recognition must be used so that the controller knows which of 
several craft in his radar or optical field of vision is in fact the one which he is 
talking to. This is an extremely serious problem when many craft are using the 
system at the same time. 

Homing or preselected course navigation often makes use of coded signals 
to indicate deviation from the preselected course. When the craft is off course on 
one side, the navigator hears coded ‘'dots", when he is off on the other, he hears 
"dashes". Similar indication is used in the radio "range" navigation system 
where the coded letter A (•-) is heard when off course on one side, and N (-•) is 
heard on the other side. A system of this type may have a two or four or multilobe 
pattern so that there are several lines along which one could navigate towards or 
away from the beacon. 

A truly general navigation system must enable the navigator to draw his 
line of position on a chart no matter where he is in relation to the station, provided 
of course that he is within range. Various systems do this by rotation or angular 
oscillation of a pattern such as the preselected course type. The navigator gets his 
line of position by aural recognition of the instant when an equisignal or otherwise 
designated line crosses his position, as in the Sonne system. 

Visual Indication 

The visual presentations of information fall into two natural classifications: 

(a) Mechanical indication, dials, pointers, or switching of lights. 

(b) Oscilloscope presentation. 

The mechanical indicators include meters of the pointer-on-scale type and 
dial indications of various sorts. They are simple and direct and have the enormous 
advantage of not presenting too much information. Zero-center meters may be used 
to indicate deviation from prescribed course or from line towards home. The fact 
that deviations right or left from course may be made to appear as right or left de- 
flections of a pointer is of great importance in that it contributes to the naturalness 
of a pilot's corrective reaction, a result not obtainable with aural presentations un- 
less some binaural device is used. Furthermore, the exact course is more sharply 
indicated and the actual extent of deviation is quantitatively displayed. This is a de- 
sirable feature since it tends to prevent hunting back and forth across the course. 
Certain systems use automatic dial reading to indicate actual lines of position; 
others depend on coincidence alignment of some type on an oscilloscope followed by 
a reading of mechanical indicators attached to the phase shifters or potentiometers 
used in adjusting the coincidence. 

Oscilloscope indicators are certainly the most versatile tools yet devised to 
present information that can be reduced to electrical or magnetic variations. The 
presentations may be of any sort from simple right-left steering directions to 
complete PPI maps of the surrounding geographical features as well as of other craft 
in the neighborhood. In general the oscilloscope presentation allows of the use of 
weaker signals than direct reading mechanical systems. Furthermore, anomalous 
propagation conditions , ref lections from tropospheric or geographic discontinuities, 
and enemy jamming or meaconing may completely upset the operation of automatic 
mechanical indicators . A skilled operator on the other hand, using an oscilloscope pre- 
sentation may recognize and discount the spurious signals and still be able to deter- 
mine his line of position. An oscilloscope presentation is particularly adapted to 
monitoring sky-wave pulses in order to select times when conditions are stable and 
hence readings are more reliable. The same might be said for any instantaneous 
mechanical type of indicator except that the oscilloscope shows not only the ampli- 


Electronic Navigation Systems 


1.39 


tude but the changes in form of the pulse which is indicative of the stability of pro- 
pagation conditions. In general it will be easier to meacon continuous -wave trans- 
missions than pulse types where the presentation is on a "scope". Suppose for 
instance that one is using a system in which lines of position are lines of constant 
radio-frequency phase difference between two carriers. A meaconing station could 
transmit one of the carrier frequencies and shift actual phase of the system locally 
or even over the entire coverage area if sufficient power were available. This could 
be done without the knowledge of navigators using the system. Also in the case of 
a system which establishes a line of position by comparison of modulation enve- 
lope phases, a continuous wave transmitter correctly phased could bring about 
a phase shift of the modulation envelope by any desired amount. Since meaconing 
cannot ordinarily be done from points near the system transmitter, the phase shifts 
produced will vary from point to point within the coverage area. Meaconing of pulse 
systems requires the generation of a false pulse. Theoretically at least it would 
not be impossible to generate a pulse which would cancel out one of the system 
pulses along an arbitrary line across the coverage area. Practically, this would be 
very difficult and the general effect would be to broaden the resultant pulse or dis- 
play two pulses. This effect might not be observed by the navigator using a mechan- 
ical type of indicator but would certainly warn him of the malicious intent of the 
enemy if he were using an oscilloscope presentation. As previously pointed out, 
with pulse transmissions the front edge of a pulse is unique in that it arrives by the 
most direct route and is not composite. If the pulse is of short duration the signals 
arriving by the different paths do not overlap and hence may be easily recognized 
on an oscilloscope. 

Oscilloscopic presentation of data has been criticized for displaying too much 
data at one instant and hence confusing the navigator. It is true that in the case of 
an inexperienced navigator, his fir St reactions to the indications on his "scope" might 
be total bewilderment, but so would the first reactions of almost any intelligent be- 
ing to the array of instruments visible from the pilot's seat of a modern plane. 
Pilots become familiar with the utility of the instruments in front of them, and navi- 
gators come to depend on their "scopes". 

In general, the mechanical indicators are best adapted to continuous -wave and 
modulated continuous -wave systems where ground- wave propagation predominates 
and where the transmissions are free from the effects of enemy machinations. 
Oscilloscope indicators are ideal for pulse types of systems where they may extend 
the useful coverage to longer ranges and enable the system to be used under less 
favorable transmission conditions. 

V. Operating Skill Required 

There is a tendency among many people to deprecate any piece of equipment 
which requires any skill on the part of the operator. If these people actually con- 
trolled the design of navigational equipment everything would have to be "foolproof" 
andnewideas would never get a trial. On the other hand, this tendency is a necessary 
and beneficial counter- balance to the imaginative genius of other people who would 
multiply and extend the intricacy of modern radar and navigational gear until only 
they themselves could operate it. The fact is that neither pilots or navigators are 
morons; they are people of well above average intelligence. They may be too busy 
under certain circumstances to be able to make careful and precise measurements, 
but given time and training, they could operate and maintain any reasonable creation 
of the electronic genius. Under the stimulation of wartime necessity, the number of 
ships and aircraft which must be navigated has multiplied enormously. The training 
of navigators must be carried out in as short a time as possible. The U. S. Navy 
allows a sixteen-week basic training period to train a man in the necessary math- 
ematics, astronomy, geography, and navigation. As a part of this training, the navi- 


1.40 


Electronic Navigation Systems 


gator learns to take a three-line celestrial "fix'* in from twenty to forty minutes, 
which is no mean accomplishment and involves the skilful use of a modern sextant, 
aninstrumentfully as delicate to use as an oscilloscope or other ordinary electronic 
gear. 


Since all navigation systems have a relatively small number of fixed trans- 
mitting stations compared to numbers of craft using the system, the training of rela- 
tively small numbers of operators and maintenance personnel for fixed installations 
is not as serious a problem as that of training the large numbers of navigators who 
will use the system in the navigation of various craft, even though the operators of 
fixed installations will in general require more extensive training than the navigators. 
The present trend in training programs is to allow as much as thirty or forty hours 
out of the sixteen-week basic training course for training in the practical operation 
of electronic navigational gear. Almost any intelligent person could learn to go 
through the motions of taking a "fix" by electronic means given sufficient time; but 
the skill and proficiency developed by training and practice enable the experienced 
navigator to get an accurate "fix" in a short time, and to be aware of faulty opera- 
tion of the gear, or anomalous propagation conditions, or enemy "tampering" with 
the signal. As previously indicated, the time for getting a "fix" must be short com- 
pared to the maximum rate at which the craft passes from one sector of a pattern 
having ambiguities to another. Another important reason for insisting on a short 
time to take a "fix" is because when an aircraft is forced down or a surface vessel 
is damaged, life or death may depend on being able to get a "fix" quickly and radio 
out the craft's position with sufficient accuracy to enable rescue to be effected or 
at least to bring rescue craft within range of a "squawker" or other portable marker 
beacon. 

VI. Equipment Required 

At the relatively few fixed stations of a navigational system such factors as 
weight, power requirements, complexity, skills required of operating and maintenance 
personnel, etc., are comparatively easy to account for. A consideration of these 
same factors poses a serious design problem for equipment on the navigated craft. 
For large surface vessels the weight and bulk of the equipment is less critical than 
for small vessels and aircraft. On single-place fighter aircraft the weight and size 
become extremely critical factors in the design, and only the simplest types of pre- 
sentation and lightest possible equipment can be tolerated. High precision and long 
range become less important than lightness and simplicity. On the other hand, ship- 
borne and large aircraft navigational aids will tend to stress precision and long 
range usefulness. For aircraft where the fuel load is a large percentage of total 
load there is a three-way balance to be achieved between weight of navigational 
equipment, accuracy of the equipment, and fuel load saved by accurate navigation. 
In other words, cutting the weight at the expense of navigational precision may mean 
that due to flying a less accurate (and hence longer) course, the weight of fuel re- 
quired increases more than the weight saved in navigational equipment. 

Fixed Station Equipment 

Since the best sites for fixed stations are usually remote from electric pow- 
er facilities, a reliable and well-regulated power plant is the first requisite of a fix- 
ed station. Except for small battery-operated beacons the power supplies are usual- 
ly generators driven by fuel-burning engines. Since the continuity and reliability of 
navigational coverage is of extreme importance, the power supply may need to have 
standby units . 

The design of radio-frequency circuits in general will be conditioned by the 
same need for continuity and reliability, and in addition must include accurate fre- 
auencv generation and phase-control circuits. 


Electronic Navigation Systems 


1.41 


The transmitting antenna array is a critical element in many systems. Care 
in the choice of sites, adequate design of grounding networks, precision of spacing 
and angular orientation of antenna arrays all contribute materially to the accuracy 
and reliability of the system. As a rule it is impractical to build a low-frequency 
vertical antenna a quarter wavelength high (this would be 1093 feet at 300 kcps and 
3280 feet at 100 kcps). Certain systems use barrage balloons to hold up a very long 
antenna but these have a foul- weather unreliability which precludes their use in a 
permanent installation. It is usually necessary to use an antenna which is a small 
fraction of a wavelength high, and then to "top load" it to increase the current at the 
base and to have the whole antenna carry a larger current, thereby increasing its 
effectiveness as a radiator. Whether the top is loaded or not, there is usually some 
type of matching circuit at the base to match the antenna to a transmission line or 
transmitter output. The whole combination constitutes a circuit with a fairly high 
effective "Q" when the actual radiating part of the antenna is short compared to a 
quarter wave length, and as a result the possible rate of rise of the front edge of a 
pulse output is limited. This is saying that the band width of the antenna and its 
associated circuits is narrow, perhaps undesirably so. Narrow-band circuits are 
used to limit transmitted radiation to a desired band width, but it is preferable to 
do such limiting and pulse shaping with more tractable circuits than antennas. Fur- 
thermore the radiation- resistance of antennas which are a small fraction of a quar- 
ter wave length (90°) is very small, being only 6.5 ohms for a 45^ antenna (1/8 
wave length) and 1.5 ohms for a 22.5® antenna. It is necessary to keep ohmic resis- 
tance of antennas and ground systems much lower than these values for any reason- 
able power efficiency. This in turn requires extensive radial grounding systems? 
Since any form of sharply directive array involves a number of antennas, it is im- 
practical to build them for low frequency systems. At higher frequencies, the anten- 
na problem is much less difficult and it is quite possible to construct antenna arrays, 
reflectors (dishes), and lenses which give sharply defined beams, if these are de- 
sired. 


VII. Frequency and Bandwidth 

A general consideration of frequency and bandwidth requirements for elec- 
tronic navigation systems involves both engineering and political aspects. The ideal 
operating frequency for a particular system and its bandwidth requirements are 
engineering problems. The availability of a given bandwidth at a particular location 
in the frequency spectrum is a political question. The possibility of accurate long- 
range navigation by electronic means has been amply demonstrated during the war, 
and the desirability of post-war maintenance of such services is obvious. A chart 
showing the present uses of various parts of the radio-frequency spectrum and the 
frequencies which the several navigation systems now use is shown on page 31.05. 
Since in most cases the exact transmitting frequency is not critical from an engin- 
eering point of view, it may be chosen to fit available ranges in the spectrum. Those 
parts of the radio spectrum which have been used for a fixed purpose for many years 
and for which large capital sums have been invested in transmitting and receiving 
equipment, as in the case of the broadcasting band from 535 to 1605 kcps, are prac- 
tically untouchable even for a worldwide navigation system. On the other hand 
amateur bands are notoriously likely to be taken over for other purposes when their 
usefulness becomes evident. Many present navigational uses for parts of the spec- 
trum are obsolescent or would be if more general use were made of present Loran 
coverage or that of any other successful long range system. 


* An excellent study of the low-frequency antenna problem under steady- state condi- 
tions is reported in ORS - P - 22-2: Loga Q 392 


1,42 


Electronic Navigation Systems 


There is plenty of room in the spectrum for a good, well*- integrated naviga- 
tion system, but there is not room for all the possible systems covering the same 
territory at the same time. However, in view of the importance of the problem it 
is highly desirable to have available spectrum space for further experimentation 
with navigation systems and hence more rapid evolution towards better ultimate 
systems. 

In considering the bandwidth of a given system it is necessary to take account 
of the bandwidth required for the complete chain of transmitting stations which are 
required to provide coverage over an ocean or a large land area such as North 
America or Europe. Thus a pulse system like Loran requires a relatively large 
bandwidth allotment, but one can operate many fixed transmitters on the same fre- 
quency since the various stations may be distinguished by their repetition rates. 
A system like Sonne on the other hand can operate with a much narrower bandwidth 
allotment per station, but cannot stack stations too closely together, and must not 
use the same frequency for any two stations which can be received simultaneously 
at any point in the coverage area. The present Loran receiver (AN/APN-4) is de- 
signedfor receiving sixteen possible pairs (sixteen repetition rates) on each of four 
frequency channels. Complete coverage of the North Atlantic would probably re- 
quire the use of all the rates on one channel. The range of Sonne is of the same 
order of magnitude as that of Loran, and sixteen Sonne stations spaced 3 kcps apart 
would require approximately 50 kcps of bandwidth for a complete system. This 
figure of 50 kcps bandwidth is of the same order of magnitude as that of an ideal 
Loran pulse transmission. Actual Loran transmissions are somewhat wider. It 
might be possible to stack Sonne transmissions as close together as 1 kcps apart, 
but sharp receiver circuits tend to broaden the equisignal and reduce the precision 
of a line of position. In general, the Germans used wider spacing than 1 kcps for 
their operating Sonne stations. The transmitted power from German Sonne stations 
was of the order of 1.5 kw which is to be compared with 100 kw of peak transmitted 
power from a typical Loran station. However, the equivalent ratio of signal strengths 
is 0.123 to 1.0. This value is indicated in relation to other signal strengths in Fig- 
ure 1-19. The average power of a Loran transmission is much less than 1.5 kw. 

Figure 1-19 shows the frequency spectra of a probability pulse like that of 
Figure 1-20 (solid line), a cosine pulse like that of Figure 1-09 and a rectangular 
pulse, all having a nominal duration of 50 microseconds. While it may be easy to 
decide what an ideal pulse should look like, it is not easy to generate such a pulse, 
modulating a radio-frequency carrier at the power levels which are necessary. The 
problem reduces to the fact that while the expenditure of considerably more money 
in engineering and operating costs might produce a better pulse from the point of 
view of reduced bandwidth of transmission, in most cases the interference produced 
in a narrow band receiver tuned 100 kcps off the center of the pulse spectrum will 
be negligible at points located at some distance from the transmitter. The distances 
at which pulses cease to be a serious source of interference depend both on the 
shape of the pulse as indicated, on the peak transmitted power, and on the local 
noise level and the noise generated in the receiver. The fact that radio noise condi- 
tions vary greatly with the location, time of day, and time of year, etc., make num- 
erical assessments meaningless except in average terms. Nevertheless the truth 
remains that while pulse forms should be improved, there is a diminishing gain to 
be achieved by carrying out extreme refinements . Beyond a certain point the only 
benefits which result are confined to very limited areas near transmitting stations. 

The design of practical pulse shapes is, like so many other engineering pro- 
blems, a compromise between a number of conflicting requirements. As pointed 
out earlier, the shorter the allowable timing uncertainty the shorter the rise time 
of the pulse must be; and again for high resolution in PPI presentations the rise 


Electronic Navigation Systems 


1.43 


-20db 


-30db 


-40db 


-50db 


-60db 



40db 


Fig. 1-19 Periodogram envelopes of rectangular, cosine, and probability pulses all 
having the same nominal duration and the same peak value. The rectangular and 
cosine pulses have harmonic terms in their periodograms which are not shown in 
the above curves since the point of interest is the rate at which the envelope de- 
creases with frequency away from the carrier. If one assumes the peak power ra- 
diated during the pulses is 100 kw then the relative field strength of a (50 kc wide) 
group of Sonne stations radiating 1.5 kw is shown for comparison. 



1.44 


Electronic Navigation Systems 



-at2 

Fig. 1-20 Comparison of a probability pulse (e = Ee ) 

e = E ( 1+ i cos /3t ) from /3t = -tt to ^ = +ir 
2 2 


and a cosine pulse 


time and total duration of pulses must be short. The ideal pulse shape from the 
point of view of minimum bandwidth required for transmission is one having the 

equation of an error function, e = Ee’^ . However, all actual pulses have to reach 
their maximum value in a finite time whereas probability pulses have infinite dura- 
tion measured along the time axis. The cosine pulse of Figure 1-09 is a simple 
mathematical form of finite time duration, and it has a reasonably narrow bandwidth 
as is evident from Figure 1-19. Many actual pulses have leading edges closely 
approximating this cosine form, although the trailing edges are more nearly of an 
exponential form. For a given pulse shape such as a half-sine or a cosine as in 
Figure 1-09, the shorter its duration in time the wider will be its spectrum and vice 
versa. However, one must be careful in interpreting the meaning of this statement. 
Consider for instance a comparison of rectangular pulses modulating a carrier and 
all having the same amplitude but different lengths in time. A receiver tuned a few 
kcps off the carrier will receive a click when the pulse begins and another when it 
stops. These clicks will have the same intensity whatever the pulse duration as 
long as it is very much longer than the period of a single RF cycle. The periodo- 
gram of such a pulse of amplitude E and having a duration 2tn is 


A{f) = 2Etn 


sm 


iil 


fn 


where f is the frequency measured from the carrier value, and In = "zr • When the 

frequency is near the carrier value, f is near zero and A(f) reduces to 2Etn, so that 
theperiodogramamplitudeatcarrierfrequency will increase with the pulse duration. 
On the other hand for f>fn, and considering the values of the periodogram at the max- 
imum points where the sine function is unity, the periodogram is given by 


Electronic Navigation Systems 


1.45 




Fig. 1-21 Modification of a rectangular 50 microsecond pulse at the end of each of 
three stages of video amplification. The upper half power frequency is 12,700 cps 
for a single stage and 6,500 cps for 3 stages in case (a); and 31,850 cps per stage 
or 16,200 cps for the three stages in case (b). The time scale is given in terms of 
the time constant of a single video stage in each case. 


1.46 


Electronic Navigation Systems 


A(f) = 


2Etn 

JL X 

2’fn 


TTf 


since 2fj^ 


J_ 

2tn 


showing that the amplitude of the periodogram is an inverse function of frequency 
off the center of the band for this particular pulse form, and is independent of the 
duration. In terms of energy, the energy in sidebands remote from the carrier fre- 
quency depends on the shape of the leading and trailing edges of the pulse, and is 
independentof pulse duration if the top of the pulse is flat, whereas the carrier ener- 
gy is a function of pulse duration. The relative amount of sideband energy radiated 
goes down as the pulse is lengthened, since it is diluted by more and more carrier 
energy, but the actual sideband energy is independent of duration and the ''clicks" 
producedare just as annoying. One solves the key "click" problem for code trans- 
mitters by simple delay circuits which round the corners of the otherwise rectangu- 
lar modulation envelope. With pulses the problem is more difficult, since the steep 
front edge of the pulse is for many uses its most important feature and one which 
must be preserved if accurate time measurements are to be made. The problem 
is to have the leading edge of the pulse arrive at its steepest portion by a gradual 
increase of slope. The probability pulse accomplishes this to the highest possible 
degree but it spends too long a time reaching its steepest part, as shown in Figure 
1-20 (solid line). The simple rule of leading and trailing edges is that if the func- 
tion of time which describes the leading or the trailing edge has a discontinuity of 
value, as in a rectangular pulse (step function), then the sideband intensities dimin- 
ish as the inverse first power of the frequency difference between sideband and car- 
rier. If the derivative of the mathematical form of the pulse has discontinuities, as 
in the case of triangular or half-sine pulses, the law is inverse square, etc. In the 
case of the cosine pulse of Figure 1-09 the first derivative is continuous but the 
second is not, so the governing function is an inverse cube. All the derivatives of 
a probability pulse are continuous. 


One might approach the problem from the point of view of physically gener- 
ating a desirable pulse by modifying a convenient initial form until it has the desir- 
ed shape. This modification may be thought of as a trimming off of the higher fre- 
quency components of the pulse. Figure 1-21 (a) and (b) shows the appearance of 
the initially rectangular pulse after each of three similar successive stages of 
video amplification for two different bandwidths. The vertical dimension of the 
pulse is kept nearly constant to bring out the essential change of shape correspond- 
ing to the reduction in intensity of the higher frequency components of the pulse 
spectrum. The last pulse shown in Figure 1-21 for each amplifier bandwidth is 
approaching the ideal probability pulse shown in Figure 1-20 (solid line). Although 
having the same initial width, the final pulse produced by the narrower amplifier 
will be broader than that produced in the wider amplifier. A similar effect is pro- 
duced in the envelope of a pulse -modulated radio-frequency signal when it is passed 
through successive tuned circuits, as in a receiver or transmitter. This is shown 
in Figure 1-22. Here again the change in shape of a rectangular pulse- modulated radio 
wave "packet" may be thought of as due to clipping the higher frequency side bands 
of the signal. After detection, this pulse- modulated signal has the appearance of the 
last pulse in Figure 1- 21 . Any measurement of the time interval between two pulses, 
or the recognition of coincidence of two pulses, will be most accurate if it can be 
made at the steepest part of the pulse. Consider the alignment of two similar pulses 
which are to be exactly superimposed as inaLoran indicator, where the pulse ampli- 
tude is vertical and the time base is horizontal. The lateral separation of two pulses 
whicharenearly superimposed is greatest at the steepest part of the pulse, as shown 
in Figure 1-23. As pointed out earlier, noise tends to broaden the trace and thus re- 
duce the precision of such a measurement or alignment by causing the two pulses 


Electronic Navigation Systems 


1.47 



Fig. 1-22 Modification of a rectan- 
gular radio-frequency pulse in a ra- 
dio-frequency amplifier having an 
overall bandwidth (for the three 
stages) of 13,000 cps (pulse length 
50 microseconds). 



1.48 


Electronic Navigation Systems 



Fig. 1-23 Showing the lateral separation of two nearly superimposed pulses 

being compared to merge into a single broad trace. As each trace is broadened the 
two pulses merge together where their slopes are small, but the last part of the 
pulses to become unresolvable is the steepest part. 

From the point of view of propagation uncertainties, the earlier in time that 
one can use the front edge of a ground- wave pulse, the less contaminated it will be 
with other modes of propagation, hence the desirability of getting up to the steep 
part as rapidly as possible. If one is using sky-wave pulses there is the further 
requirement that the ground- wave pulse must not be so long that its trailing edge 
contaminates the leading edge of the desired sky-wave pulse. Thus both the steep 
front edge and the short tail are necessary attributes of practical pulses for such 
comparisons as are made in a Loran indicator. For any required steepness, which 
isusually numerically given in ter ms of rise time, one could stipulate the necessary 
duration of a pulse for any choice of pulse form (such as cosine or probability) and 
hence the band width required for transmission. Since the pulse which is finally 
displayed on the receiver scope is shaped by all the circuits through which it passes, 
a natural question arises as to relative bandwidths of transmitted pulse and receiver 
response. It is fairly obvious that there is no reason for having the receiver band- 
width much broader than that of the pulse being received, since the pulse shape is 
not improved (made steeper or shorter) by this procedure and the broader receiver 
picks up more noise. For any received pulse, the receiver will always increase the 
rise time and broaden the pulse by an amount inversely proportional to the overall 
receiver bandwidth. Figure 1-24 shows the effect on a cosine pulse, of the form 
used in Figure 1-09 and having a duration of 50 microseconds at half amplitude, 
produced by single-stage video amplifiers of three different bandwidths (30 kcps, 
20 kcps, and 15 kcps). After passing through the 30-kcps amplifier, the pulse is 
broadened somewhat, but if one used a broader amplifier than 30 kcps the improve- 
ment in pulse form would be slight for the added cost and increased noise reception. 
On the other hand the pulse after the 15- kcps amplifier has nearly twice its original 
base lengthof 100 microseconds. There is an approximate relationship between the 


RELATIVE AMPLITUDE 


Electronic Navigation Systems 


1.49 



Fig. 1-24 Distortionof a cosine-shaped pulse in a single-stage amplifier, for three 
different bandwidths, showing the upper envelope of the pulse only. 


1.50 


Electronic Navigation Systems 


rise time of a pulse and the upper half-power frequency of a video amplifier which 
is capable of transmitting the pulse with negligible distortion; i.e., 

0 35 

the rise time (seconds) = r— r? r ? r 

upper half power frequency (cps) 

A similar relation holds for a radio-frequency amplifier transmitting a pulse-mod- 
ulated radio-frequency signal; here 

0 7 

the rise time (seconds) = r ; — tVtt— t v 

bandwidth (cps) 

where the bandwidth is the frequency difference between upper and lower half-power 
frequencies for the amplifier. The approximations are due to the fact that video 
and RF amplifiers may be compensated or aligned in various ways and the numer- 
ical factor depends on the amplifier circuits as well as on the particular pulse 
shape and the definition of negligible distortion. The ability of a receiver to fol- 
low a rapidly changing modulation envelope, whether it be a pulse modulation or 
a sinusoidal modulation, is the basic property which is measured numerically 
in terms of bandwidth. A similar statement holds for the ability of a video am- 
plifier to follow a rapidly varying signal. The word "follow" used here refers 
both to following up the rise and down on the trailing edge. There is a tendency 
in present equipment to have the overall receiver bandwidth rather narrower than 
that of the transmitted signal, in order to improve the signal-to-noise ratio. If 
such a receiver operates satisfactorily from the point of view of precision of re- 
sult, the implication is that the transmitted signal is broader than it needs to be and 
it should be tailored to reduce the required bandwidth for transmission. The whole 
question of ideal signal-to-noise ratios depends on the type of transmission, and the 
presentation anduse of the received signal. In search radar applications, where the 
primary object is to detect the presence of craft at extreme ranges, it is desirable 
to use long pulses and narrow receivers. The presentation will show relatively 
large blobs for targets, incapable of fine resolution, but easier to spot above the 
noise which clutters the screen and hides the target whose location is unknown. On 
the other hand a broader receiver and shorter pulse could be used in the case of a 
target which has been located and enclosed within a narrow "gate" so that only those 
noise pulses which arrive within the time duration of the "gate" are seen along with 
the desired signal. 

In the matter of signal-to-noise ratio, the various continuous -wave systems 
have the advantage of being able to use quite narrow band receivers. Changes in 
craft position are so slow, even at airspeeds contemplated, compared to the rapid 
changes of instantaneous radio-frequency voltage in a cycle or a modulation enve- 
lope, that one received cycle is almost identical to the next or to the next hundred 
cycles, andas a result the bandwidth required is small. However, it must be borne 
in mind that if radio-frequency phase is the information- bearing variable, then the 
amplifier must be stable against phase shifts. The narrower the bandwidth, the 
more sensitive a receiver is to slight frequency drift. The angular phase shift 
caused by a slight frequency change in received radiation or drift of receiver reso- 
nant frequency is given for a single-stage single-tuned receiver by 

AO = 

Bandwidth (cps) 

where AO is the uncertainty in phase angle, Af (cps) is the maximum deviation of 
receiver frequency from signal frequency. This difference is assumed small in 
comparison to the bandwidth and constitutes the frequency uncertainty. For sever- 
al stages, the total phase shift is the sum of the phase shifts for each individual 


Electronic Navigation Systems 


1.51 


stage. The overall bandwidth diminishes as the number of like stages increases, 
but not in simple inverse proportion. So that in general this formula gives too small 
a value for phase angle uncertainty in terms of overall bandwidth for a number of 
single-tuned stages. It is possible to design double- tuned stages and combinations 
of double- and single-tuned stages which are more stable against phase shifts. 
In general the more specialized the receiver the more difficult is its operation, 
service, and maintenance. As an example, suppose the bandwidth is 100 cycles per 
second and that it is required to keep uncertainties in phase below 3.6 degrees, or 
0.0628 radians, then the maximum allowable frequency drift will be 3.14 cycles per 
second which is one part in 10^ at 300 kcps. This would be relatively easy at the 
transmitting station where accurate crystal control of frequency is possible but 
would be more difficult in a receiver where thermal drift of the inductance and 
capacitance elements in the tuned circuits is not easily controlled under operating 
conditions which involve large temperature variations and require light weight and 
small physical dimensions. The Decca system requires phase-stable amplifiers 
and on the assumption that Decca has a range as great as Sonne or Loran, it would 
need 32 phase-stable amplifier channels for a receiver to use a chain of stations 
covering the North Atlantic for instance. There is the further difficulty that these 
32 frequencies must all be submultiples of some higher frequency and it might be 
rather difficult to find space in the spectrum for a group of frequencies covering 
such a large overall band spread, even though the individual channels are very nar- 
row. For a system like the Federal long-range system, the relative amplitude of 
received signals is the quantity which bears the information. Here the very nar- 
row amplifier must be stable and linear with respect to input signal amplitude, since 
signals of different strengths have to be compared. 


VIII. Present Status 

There is a wide difference between an operational system and a proposal. 
When a system has been proposed, there is a temptation to dismiss the working out 
of practical details as if the success of such a process were a foregone conclusion. 
When one examines the detail of some of the operational systems it is obvious that 
more than one stroke of genius has gone into the working out of this detail. 

On the other hand the operational systems have not reached their ultimate 
state of perfection and systems still require integration into unified navigational 
utilities. 


j 


I 

] 


i 


Beacons and Interrogators 


2.01 


Part I General Information 
Introduction 

A radio beacon is an installation of radio transmitting, or receiving and 
transmitting apparatus which supplies suitable information for use in the deter- 
mination of one or more of the following: range, azimuth, identification. The 
following discussion is concerned mainly with radar beacons or racons of the so- 
called responder type which automatically transmit a reply signal only upon the 
reception of an interrogating signal consisting of a radar pulse of a length special- 
ly reservedfor this function alone. This feature adds a measure of security to the 
operation of the racons. Responder beacons may be interrogated either by regular 
search radar sets (using a special pulse length of from 2 to 5 microseconds dur- 
ing beacon operation) or by so-called interrogator-responser installations designed 
especially for the purpose of interrogating beacons and receiving the beacon re- 
sponse signals. Responder beacons used for purposes of IFF, are commonly re- 
ferred to as transponders. Interrogator- responsers are often referred to simply 
as interrogators ; and these installations may or may not be synchronized with a 
local radar set. Interrogator- responsers which are synchronized with local radar 
sets usually operate at a submultiple of the radar pulse repetition rate in order to 
reduce the likelyhood of overinterrogation of the radar beacon which may be re- 
sponding to a number of interrogators simultaneously. Beacons operating in the 
super-high-frequency X or K bands are limited to the use of relatively heavy trans- 
mitting equipment consisting of magnetrons and wave guides, while those operat- 
ing in the lower frequency bands may utilize ordinary vacuum tubes and lumped 
constant circuits which in general are much lighter in weight. Both magnetrons 
and UHF triodes are used in the S-band. 

Uses 

Radar beacons have a number of uses, the most important of which are: 
(a) fixed ground installations for general navigational use by aircraft, (b) portable 
and mobile beacons for temporary navigation (including homing). (c) airborne 
beacons for identification and control of aircraft both within and beyond normal 
radar range. 

Triggering Requirements 

The requirements of a beacon depend to a certain extent upon its particular 
use. With the exception of some IFF transponders, a beacon is designed so that 
it may be triggered or interrogated directly by pulses from radar sets operating 
anywhere within a particular frequency band such as the X-band or S-band. Most 
racons respond only to pulses having a time duration of between 2 and 4.5 micro- 
seconds, and are unaffected by normal radar search pulses most of which have a 
duration of one microsecond or less. Increased security may be obtained if nec- 
essary by the use of beacons requiring two interrogating pulses occurring simul- 
taneously on different frequencies. 

A radar responder beacon replies on a different frequency from that of the 
radar interrogation. In order to eliminate ground clutter and other radar echoes 
when observing beacon responses on a PPI, the receiver of the radar set is made 
sensitive to signals of the beacon response frequency and insensitive to the echo 
signals of the radar transmission frequency. For example, the standard X-band 
beacon response frequency is 9310 mcps which is just below the 9335 to 9415 mcps 
range of the standard aircraft X-band radar frequencies. A separate local oscilla- 
tor is usually provided in the radar receiver for reception at the beacon response 
frequency. During reception at the beacon frequency, a loss which may amount to 
as much as 20 db occurs due to the presence of the TR switch which acts as a 
relatively high-Q band-pass filter tuned for best reception at the frequency of the 
radar transmitter rather than at the beacon response frequency. However in order 


2.02 


Beacons and Interrogators 


Code: Narrow-Wide — Narrow- Wide (nwnw) 




CODE* null is on indicator at all times 
(C) 

Fig. 2-01 Types of beacon codes 


Beacons and Interrogators 


2.03 


to reduce the high TR loss, radars of recent design have incorporated a relay-oper- 
ated device for automatic retuning of the TR box to the beacon frequency during 
beacon reception. 

Coding 

Beacon response signals may be coded for purposes of identification. Sever- 
al types of coding are in use, the most common of which is known as range coding. 
Some IFF equipment however, makes use of either gap or sequence coding neither 
of which gives instantaneous identification as does range coding. The three types 
of coding are illustrated by the diagrams and cathode ray tube indications shown in 
Figure 2-01. 

Sequence coding is illustrated in Figure 2-01(aJl Several hundred consecutive 
responses appear as a single pip on the A scope lasting for about one half second, 
following which is a 2.5 second interval in which no signals are returned by the 
beacon and only the main bang is visible on the scope. Then the beacon responds 
again for one half second but this time returning wider pulses which show up as a 
wider response signal on the A scan. Groups of wide and narrow response pulses 
separated by longer no-response intervals follow one another in some definite 
sequence to form a code such as the narrow-wide-narrow- wide code illustrated in 
Figure 2-01(a). 

Gap coding, illustrated in Figure 2-0l(b)is similar to sequence coding in that 
considerable time is required in order to read the complete code from the oscillo- 
scope. In gap coding the beacon response consists of a continuous series of identi- 
cal pulses with occasional brief interruptions or gaps which are arranged to form 
a code such as the Morse letter U illustrated in Figure 2- 01(b). A complete code 
re-occurs about every 30 seconds and consists of some combination of short and 
long reply periods separated by gaps which are short compared to those immediate- 
ly preceding or following the code. 

As illustrated in Figure 2-01(c) range coding gives immediate identification 
because the entire code appears at once on the screen of the oscilloscope. The 
beacon response to a single interrogating pulse consists of a series of from two 
to six appropriately spaced pulses. The spacing between any two pulses is usually 
either about 15 or 35 microseconds corresponding to range increments of about 
1.2 or 2.9 nautical miles respectively. Both the number of pulses and the spacing 
between them may be varied to form different codes. On the screen of the oscillo- 
scope the distance to the first beacon response signal is indicative of the range between 
the beacon and the interrogating radar. 

Range coding can be read on intensity- modulated oscilloscopes (B or PPI 
scans) more easily than can either sequence or gap coding. Range codes are usually 
produced by an electronic coder, while sequence or gap codes are usually produced 
by mechanical coders. 

Many interrogator- responser units of the type used primarily for homing 
operations make use of the L-scan type of presentation. The L-scan is sometimes 
referred to as a double A-scan. Ranges are indicated on the vertical scale and 
signal blips appear horizontally on either or both sides of a vertical center line 
as shown in Figure 2-02. Right and left steering directions for homing are usually 
indicated by inequality of amplitude of the signal blips appearing on the two sides 
of the scope presentation. This may be accomplished by simple lobe- switching 
technique aboard the navigating craft. Course or track indications may also 


2.04 


Beacons and Interrogators 


be obtained if lobe- switching is also em- 
ployed at the fixed ground beacon in- 
stallation. (See Section 24). 

Nature of Response Signals 

As viewed on a PPI scan, a beacon 
response normally takes the form of a 
small arc similar to that produced by a 
radar echo signal from a large isolated 
object. However, there is considerable 
variation in the effective angular width 
of the radar beam for both interrogation 
and reception. As a radar beam sweeps 
over a beacon location, the time or angu- 
lar spread within which the signal strength 
at the beacon site is sufficient to trigger 
the beacon depends upon the horizontal 
field pattern of the radar beam, and the 
maximum field strength at the beacon site, 
andalsouponthesensitivity of the beacon receiver and the directivity characteristics 
of the beacon receiving antenna. The angular spread of the visual signal on the 
PPI tube depends upon all of the above and also upon the sensitivity and directivity 
characteristics of tl^ radar receiving system. The angular spread for interro- 
gation varies from 0^ at maximum range to a full circle at close range. Between 
the maximum range and about 1/10 of maximum range, only the main lobe of the 
radar beam is sufficiently strong to interrogate the beacon. Below about 1/10 of 
the maximum range the side lobes may also interrogate the beacon, so that at close 
range the beacon response may appear as a full circle on a PPI scan, unless the 
directivity of the radar receiving antenna is sufficient to limit the angle through 
which the beacon response may be seen. It is thus quite apparent why beacon re- 
ceivers should not be too sensitive. At close range the angle of interrogation may 
in some cases be reduced if an aircraft radar antenna is tilted upward to reduce 
the field strength at the ground beacon. The fact that most ground beacons have 
a rather sharp vertical directivity pattern beamed on the horizon helps somewhat 
in reducing the received signal strength at close range from aircraft at high 
altitudes. In fact the beacon response may even be lost entirely by an interrogating 
aircraft nearly over the beacon. 

Radar beacon response pulses are usually of the order of 0.5 microsecond 
duration. Since 0.5 microsecond corresponds to the time required for an electro- 
magnetic wave to travel about 1/10 mile, beacon response pulses of 0.5 microsecond 
duration often do not show up very clearly on a 100- mile sweep even though the 
video signal strength is adequate. The visibility of a beacon response on the PPI 
screen may be improved by "video stretching". The video stretching feature 
consists of lengthening the 0.5 microsecond video pulses to about 2.5 microseconds 
duration by means of an appropriate circuit. 

Overinterrogation 

The transmitter of a responder beacon has a limited traffic handling capacity. 
After an interrogating pulse has been accepted by the beacon discriminator circuit, 
the beacon receiver is made insensitive to further interrogation pulses for a period 
of about 200 microseconds. If a number of radars are working a responder beacon, 
a fraction of the interrogating signals sent out from each radar will go unanswered 
because they arrive at the beacon less than 200 microseconds after accepted sig- 



Fig. 2-02 Example of L-scan 
presentation 


Beacons and Interrogators 


2.05 


nals from other radars. The code arcs on a PPI scan will then appear slightly 
broken up. If so many radars interrogate the beacon that its transmitter load 
reaches its safe upper limit, the period during which the beacon receiver is in- 
sensitive is automatically lengthened to a value sufficient to prevent further increase 
in the transmitter load. The beacon replies are shared statistically among the 
interrogating radars so that each radar will always receive some replies as long 
as its repetition rate is not synchronized with that of any other interrogating radar. 
Exact synchronization of repetition rates is a most unlikely occurrence. A micro- 
wave beacon such as AN/CPN- 6 can serve as many as 50 to 100 aircraft. The num- 
ber of aircraft which can simultaneously interrogate a beacon and receive intelligible 
responses signals depends upon the types of beacon and interrogating radars used. 

Accuracy 

The accuracy of measurement of the range or bearing of a beacon depends 
largely upon the interrogating radar equipment. Radars equipped with 10 to 15- 
mile sweeps with step range- delays can give the apparent slant range to within 
one- tenth mile or less. A correction of from 0.5 to 0.6 mile must be subtracted 
from the apparent slant range to compensate for delay in the beacon circuits. The 
measurementof azimuth or bearing of aground beacon by an aircraft radar is limit- 
ed to an accuracy of one or two degrees corresponding to an angular position error 
of 2 to 4 miles at a range of 100 miles. Since range can be measured with greater 
precision than azimuth, a more accurate navigational fix can in general be made 
by simultaneous measurementof the ranges to two beacons rather than by measure- 
ment of both range and azimuth of a single beacon. 


Range and Siting of Microwave Beacons 

Very-high-frequency radio waves travel in nearly straight lines, so that 


the maximum range at which 
by simple geometry. For 



a microwave beacon can be seen , may be determined 
example, in Figure 2-03, the line BP tangent to the 
earth's surface illustrates the maximum 
possible range for the case of an aircraft 


at an altitude Hp interrogating a ground 
beacon the antenna of which is at a height 
above the sea level surface of the earth. 
The maximum range between aircraft and 
beacon antenna is the sum of the respec- 
tive "horizon ranges" R^j and Rp corres- 
ponding to the heights and Hp . T he aver- 

age radar- horizon range, R, in nautical 
miles from any altitude, IL in feet is given 
by the formula R = 1.22VH. The above 
formula yields a value of R which is about 
1 5 per cent larger than the geometrical 
horizon range illustrated in Figure 2-03, 
in order to allow for the average amount 
of bending (or refraction) of the tangent 
radio beam. To save numerical calcula- 
tion, the chart of Figure 2-04 is provided 
for the determination of average radar- 
horizon ranges. The chart also contains in graphical form, information for the 
determination of the approximate total range between aircraft and ground beacon 
for the case in which an obstruction such as a building or hill causes the effective 
skyline to be at a small angle of elevation above the sea- level horizon. The number 
of angular degrees marking the dashed lines of Figure 2-04 correspond to the angle 
of elevation "A" above sea level as shown in Figure 2-05 or as shown with some- 
what less distortion in Figure 2-06. 


for porpoMi of llluotrotlon tho holght* and H, fcar* 
boon sroatly rolatlTo to tho oarth'a radlua. 


Fig. 2-03 Horizon- range diagram 


HEIGHT IN FEET 


2.06 


Beacons and Interrogators 



RANGE IN NAUTICAL MILES 

Fig. 2-04 Average radar-horizon range characteristics 

The chart of Figure 2-04 is useful 
for the rapid determination of expected 
range vs. aircraft altitude charts for any 
beacon site. A skyline survey made with 
a good transit located at or very near to 
the beacon antenna yields the angle of ele- 
vation of important skyline features as a 
function of azimuth angle. The results of 
such a skyline survey might appear as 
shown in Figure 2-07 which is an imagin- 
ary set of data drawn to represent a typi- 
cal seaside beacon having open sea from 
about 0® to 180® azimuth. Due to the ele- 
vation of the beacon antenna, the sea- level 
horizon- angle is in this case - .18®. The 
angle A in Figure 2-06 is the angle of 
elevation measured above the sea- level 
horizon and would be .30 + .18= .48 degrees 
for the highest point H to the northwest in Figure 2-07. At an inland beacon site 
the depression angle of the sea level horizon may be calculated from the formula: 
Depression angle d = 0.01 8 degrees with in feet above sea level. The height 
of the beacon antenna used in the above numerical example was assumed to be 100 
feet. With the aid of the chart of Figure 2-04, the data from the skyline survey may 



Fig. 2-05 Reduction of maximum 
range by skyline object ”0" 



DEGREES ABOVE OR BELOW HORIZONTAL 



Fig. 2-06 Details of the angle of elevation A 

be readily converted into a useful expected range vs . aircraft altitude chart such as 
that of Figure 2-08 for the given beacon. Maximum ranges for microwave beacons 
may be predicted by the above method more accurately than they can be checked 
without many hundreds of carefully controlled test flights for each beacon site. 

As an aircraft approaches a beacon within the horizon range, interference 
effects may occur in the form of a slow recurrent fading of the beacon signals. The 
fading is due to partial cancellation between a direct signal and a signal reflected 
from the earth' s surface. Overland, the fading due to interference effects is usual- 
ly slight, but reflections from a water surface may cause the fading to amount to 20 
db or more. 

As long as an aircraft is above the radar horizon, the received signal power 

^ Q Pt Gp Gb 

is given by the formula: P^. = 1.85 x 10"^^ ^^2 watts where Pj. is the pow- 

er transmitted, Gp and Gb are the absolute gains of the aircraft and beacon antennas 
respectively, X is the wavelength in centimeters, and R is the range in nautical 
miles. Either leg of the transmission - from aircraft to beacon or from beacon to 
aircraft - may be the limiting factor in determining the maximum usable range. An 
aircraft flying at an altitude of 30,000 feet should be able to obtain responses from 
beacons at ranges up to about 230 nautical miles (radar horizon). Beyond the radar 



Fig. 2-07 Graph showing the elevation of the skyline from the beacon in the 
various directions of the compass. (Beacon height 100 feet) 


2.08 


Beacons and Interrogators 



s 

Fig. 2-08 Average beacon range in nautical miles for 
aircraft at elevation shown 

horizon however, the signal strength drops off very rapidly and even a large in- 
crease of power would increase the attainable range by only a few miles. 

Part II Description of a Typical Beacon (AN/CPN-6) 

A description of a typical radar responder beacon follows: AN/CPN-6 is a 
heavy X-band radar responder beacon designed for ground, ship, or truck installa- 
tion. An operational block diagram of this beacon is shown in Figure 2-09. 

When a radar operator in an aircraft wishes to obtain a beacon signal, he 
turns a selector switch from SEARCH to BEACON. This changes his radar trans- 
mitter pulse width and receiver tuning so as to make possible interrogation and 
reception of the beacon. 

The RF interrogating signal from the radar transmitter enters the omni- 



Beacons and Interrogators 


2.09 



RECEIVER CABINET TRANSMITTER CABINET 

Fig. 2-09 Operational block diagram of X-band beacon AN/CPN-6 

directional beacon receiving antenna and is led by wave- guide to the superhetero- 
dyne receiver. If the interrogating signal frequency lies within the receiver pass- 
band extending from 9320 mcps to 9430 mcps, it is converted to an intermediate 
frequency and amplified, converted to a "video" pulse envelope, amplified again and 
led to a pulse- width discriminator circuit. The discriminator circuit rejects ordin- 
ary search pulses shorter than two microseconds, or longer than five microseconds 
and passes beacon interrogation pulses of between two and five microseconds dura- 
tion. A pulse which has passed through the discriminator circuit triggers a self- 
blanking multivibrator and also the first of a series of multivibrator circuits in the 
coder. The self-blanking multivibrator keeps the coder insensitive to incoming 
signals for a time somewhat longer than the duration of the code train. This limits 


2.10 


Beacons and Interrogators 


the maximum duty cycle and also prevents the beacon transmitter from triggering 
itself in a "ring around" fashion. The coder forms a series of from two to six 
trigger pulses with either short (15 microseconds) or long (35 microseconds) spaces 
between them, depending upon the setting of the code selector switches. The code 
trigger pulses are next sharpened, reduced in length to one-half microsecond, and 
amplified by the modulator driver. In the modulator the pulses are squared-up, 
further amplified to 11,000 volts peak, and applied to the magnetron transmitter 
operating at the X-band beacon frequency of 9310 mcps. A resonant cavity is used 
to tune the magnetron and stabilize its output frequency. The RF output of the mag- 
netron is led by wave- guide to an omnidirectional transmitting antenna. 

The beacon response signal is picked up by the radar set in the interrogat- 
ing aircraft and displayed on the PPI screen as in Figure 2- 01(c). The distance 
from the center of the PPI to the nearest small curved arc gives the slant range be- 
tween beacon and aircraft (assuming an undelayed sweep). The series of arcs or 
dashes forms the code which identifies a particular beacon. The azimuth bearing 
of the beacon is read off the PPI in the same manner as for radar signals. 

A brief description of some of the component parts of the racon system fol- 
lows: 

Antennas and RF Lines 

A rectangular wave-guide 1x1/2 inch, is used for the transmission of RF 
energy because of its mechanical simplicity, low loss, and high power- handling ca- 
pacity. Stub transformers are used at the base of each antenna unit to convert from 
the normal mode in the rectangular guide-feeder to the second mode (TMq^) in the 
cylindrical guide of the antenna or vice versa. Both transmitting and receiving an- 
tennas are of the slotted, cylindrical, wave- guide type giving horizontally polarized 
radiation with an omnidirectional pattern in the horizontal plane, and a half-power 
vertical beam width of five degrees centered on the horizon when the antenna axis 
is in a vertical position. The transmitting and receiving antennas are virtually iden- 
tical, but the transmitting- antenna array is tuned for best operation at the beacon 
frequency of 9310 mcps while the receiving array is adjusted for best response at 
9375 mcps, which is the center of the receiver pass band. For shipboard installa- 
tions special antennas are provided which have a vertical beam width of 30 degrees 
at the half-power points, corresponding to a gain of only 3 compared to a gain of 20 
for the 5 degree beam. This sacrifice of nearly 10 db in beacon performance is 
necessary in order to obtain such a broad beam that the beacon response will remain 
visible when the ship is rolling heavily. 

The output frequency of the beacon transmitter is checked by a transmitter 
frequency monitor consisting of several components. A directional coupler extracts 
one part in 20,000 from the RF power output of the beacon transmitter. The direc- 
tional coupler feeds a resonant cavity built of low temperature coefficient alloy, pre- 
tuned at the factory to the desired beacon frequency, and having a Q factor of about 
10,000 sothatits transmission is down fifty percent at plus or minus 0.5 mcps from 
the resonant frequency. The resonant cavity is followed by a crystal detector, 
amplifier, and indicating meter. The magnetron frequency puller (stabilizing tun- 
er) is adjusted until the indicating meter of the frequency monitor reads a maximum. 
A similar receiver frequency- monitor is also provided for use in adjusting the 
local oscillator circuit. 

Oscillator and IF Amplifier 

A racon has the difficult job of responding to signals from radar sets whose 
transmitter frequencies lie scattered throughout a rather wide frequency band of 
approximately 110 mcps width. The actual bandwidth of the IF amplifier used in 


Beacons and Interrogators 


2.11 


this receiver is about 35 mcps extending from approximately 11 to 46 mcps. The 
required 110 mcps bandwidth is obtained by utilizing both sum and difference fre- 
quency components in the output from the converter, and by shifting the local oscil- 
lator frequency back and forth between two fixed values chosen such that the ampli- 
fied radio-frequency bands are periodically shifted and overlap slightly to produce 
a wide effective RF pass band as illustrated in Figure 2-10. The switching of the 
local oscillator frequency takes place at a rate of between 150 and 200 times a sec- 
ond. An RF signal not lying in a region of overlap would of course be amplified 
only half of the time. 




R-F BANDS COVERED WITH LOCAL OSCILLATOR 
FREQUENCY AT ‘'F;"0R 9360 MCPS. 


y///A R-F bands covered with local oscillator 

////X FREQUENCY AT “F^" OR 9390 MCPS. 


Fig. 2-10 Receiver band covered by switching frequency of local oscillator 



ELECTRONIC SWITCHING 


TUNING CONTROLS 


TO 

PULSE WIDTH 
DISCRIMINATOR 


Fig. 2-11 Receiver block diagram (AN/CPN-6) 



2.12 


Beacons and Interrogators 


R.F. 

IJIPUT 



Fig. 2-12 Electronic switching circuit, local oscillator, and crystal mixer 

(AN/CPN-6) 



Fig. 2-13 Power output and frequency characteristics of a typical 

723-A/B tube 




Beacons and Interrogators 


2.13 


A block diagram of the AN/CPN-6 receiver components is shown in Figure 
2-11. The local oscillator- tube (type 723- A/B) is a velocity- modulated tube of the 
reflex type. Its frequency is caused to vary in jumps by application of a rectangular 
voltage to its reflector electrode, as indicated in Figure 2-12. The rectangular 
voltage waveform is generated by a multivibrator circuit and coupled to the local 
oscillator through a cathode-follower switching- tube which conducts for half the 
cycle. The voltage of the reflector electrode of the 723- A/B tube oscillates be- 
tween two levels ; a negative voltage set by the DC level control during the nega- 
tive half of the multivibrator square-wave cycle, and a less-negative voltage de- 
termined by the setting of the switching- level control during the positive half of 
the multivibrator cycle. Frequency and power output characteristics of the local 
oscillator are shown in Figure 2-13 as a function of the ref lector- to- cathode vol- 
tage. A mode such as the second from the left may be moved by mechanical tun- 
ing of the tube's resonant cavity until the center of the mode B is at about 9375 
mcps which is the desired center-frequency of the receiver pass band. The 
switching of the local oscillator is adjusted by the DC level and switching controls 
so that half of the time the operating conditions correspond to point A (9390 mcps) 
and the other half of the time to point C (9360 mcps). A probe couples the cavity 
of the local oscillator tube to a section of wave guide containing the non-linear 
crystal mixing element, the output of which is led to the IF strip. 

The IF amplifier- strip consists of eight identical stages the first of which 
and its input circuit from the crystal mixer are shown in Figure 2-14. Each IF 
stage is essentially a wide band video amplifier with series-shunt peaking com- 
pensation. The gain at frequencies below ten megacycles is greatly reduced by a 



Fig. 2-14 IF input circuit and first IF amplifier stage 



2.14 


Beacons and Interrogators 


cathode degeneration circuit in order to obtain a band-pass characteristic extend- 
ing from approximately 11 to 46 mcps. A typical overall response curve for the 
IF strip is shown in Figure 2-15. 

The IF amplifier stages are followed by a diode detector which feeds the 
envelope pulse to three conventional video- amplifier stages the output of which goes 
to the pulse-width discriminator circuit. 



Pulse-Width Discrimination 

The pulse-width discriminator circuit is shown in Figure 2-16. The output 
of the final video- amplifier stage is applied to a diode clipper- circuit which passes 
the negative portion of a signal pulse to the grid of the unbiased “drooler" tube. 
This negative signal drives the drooler well beyond plate current cut-off and its 
plate potential then rises exponentially toward +260 volts as its plate- to- ground 
capacitance charges through the 270,000 ohm plate load resistor. The rise of plate 
potential, the first part of which is quite linear, is coupled to the grid of a cathode- 
follower which is normally biased beyond plate current cut-off. The plate circuit 
of this tube contains an inductive plate load which presents a significant impedance 
only to very rapid variations in plate current; but since this tube normally acts as 
a cathode-follower most of the time, it is referred to as such. The bias on the 
cathode-follower is adjusted (by the control labelled 2 MICROSECONDS in Figure 
2-16) so that approximately 1.9 microseconds is required for the positive- going 
signal applied to the grid to raise its potential to the point at which plate current 
starts flowing. Then the plate potential of the cathode- follower drops to a lower 
value with the flow of plate current. If now the input pulse from the video stage 
should terminate at say 2.0 microseconds after its start, the drooler tube imme- 
diately becomes conducting and drives the cathode-follower beyond plate current 
cut-off. The sudden stopping of plate current in the inductive plate- circuit of the 
cathode-follower generates a positive pulse to trip the biased blocking- oscillator 
which in turn sends a negative triggering pulse to the coder. If however, a signal 
pulse from the video amplifier is shorter th^ about 1.9 microseconds, the cathode- 
follower never becomes conducting and no triggering pulse is sent to the coder. The 
circuit thus far described discriminates against pulses of shortei‘ duration than 
about 1.9 microseconds (most radar search pulses are of the order of one micro- 
second or less) and allows pulses of about two microseconds or longer to trigger 
the coder. 



Beacons and Interrogators 


2.15 



I 


\ 

I 



— II o 

.001/if 



+250W. 


Discriminator 
Output 
to Coder 


Fig. 2-16 Pulse width discriminator circuit 


It is sometimes necessary to simultaneously discriminate against pulses 
longer than about five microseconds as well as shorter than two microseconds. 
This is accomplished in the circuit of Figure 2-16 by applying to the blocking 
oscillator grid a delayed negative signal of sufficient magnitude to completely 
overpower any positive pulse from the plate of the cathode-follower occurring upon 
the termination of a signal pulse longer than about five microseconds duration. 
During a long signal pulse the voltage at the cathode of the cathode- follower con- 
tinues to rise. This voltage is coupled to the grid of an amplifier with sufficient 
negative bias to prevent the flow of plate current until five microseconds after the 
start of the positive- going signal from the cathode-follower. When plate current 
starts to flow in the biased amplifier, its plate potential drops and a negative gate- 
like signal is impressed on the grid of the blocking oscillator. Upon the termination 
of a long video-signal pulse, a positive trigger from the plate of the cathode-follower 
is applied to the grid of the blocking oscillator. The biased amplifier also sudden- 
ly stops conducting and its plate potential rises. However the plate- to- ground 
capacitance of the biased amplifier stage and also capacitor C-312 (see Figure 
2-16) take sufficiently long to charge that there is still a negative signal at the grid 
of the blocking oscillator sufficient to prevent its triggering. 

Formation of Code 

Each output pulse from the discriminator triggers the coder, the function of 
which is to form a series of from 2 to 6 pips spaced 15 or 35 microseconds apart 
for use in controlling the range-coded beacon response. A schematic circuit dia- 
gram of the coder appears in Figure 2-17. It consists essentially of a chain of 
single-shot multivibrators, firing in sequence, the cycle of events for any one multi- 
vibrator being initiated at the close of the cycle for the previous stage. In each 
multivibrator a sudden drop of plate potential is differentiated to obtain sharp neg- 
ative pips at the input to the collector- amplifier V-407. The grid resistor of the 
collector tube forms a differentiating circuit with the coupling capacitors from the 
output of each multivibrator. The response of the differentiator to the more slow- 
ly rising part of the plate voltage waveforms is negligibly small as shown by volt- 
age waveform 07 of Figure 2-18. 


2.16 


Beacons and Interrogators 


Briefly the operation of the coder is as follows: The negative trigger pulse 
from the discriminator circuit trips a "single shot" multivibrator circuit which 
generates a self- blanking gate that prevents further triggering of the coder for a 
period ranging from a normal length of about 175 microseconds to a maximum of 
about 1800 microseconds whenever too many aircraft are seeking replies from the 
beacon. Simultaneously with the triggering of the blanking gate, a negative pulse 
passes to the "collector" through capacitor C-407. At this instant the first pip- 
forming multivibrator is also triggered and either 15 or 35 microseconds later 
(depending upon the position of the first spacing control switch) this multivibrator 
delivers a second negative pulse to the collector through C-408. Each of the Re- 
maining pip-forming multivibrators then fire, one after another, to form the re- 
mainder of the code. The spacing between any two pips may be set to either 15 or 
35 microseconds by changing the capacitive part of the time constant in the appro- 
priate multivibrator. A selector switch is also available which grounds a grid of 
any of the last four multivibrators in order to provide for termination of the code 
at fewer than six pips. A few typical voltage waveforms are given in Figure 2-18. 

The circuit used for protection against overinterrogation works in the follow- 
ing way: The first tube of the coder is a single- shot multivibrator which starts the 
operation of the coder when the grid of its normally conducting section V-401B is 
driven beyond cut-off by a negative pulse from the video amplifier. While the grid 
of V-401B is negative, the multivibrator is of course insensitive to further negative 
triggering pulses so that during its period of operation it generates its own blank- 
ing gate. A DC voltage from the transmitter circuit directly proportional to the 
magnetron cur rent is fed to the grid of a biased DC amplifier tube (V-408 of Figure 
2-17) which is non-conducting at low interrogation rates. When the average interro- 
gation rate exceeds a certain level, V-408 conducts a current which passes through 
the bias and cathode- level control- circuits of the self- blanking multivibrator V-401, 
the effect of which is to increase the plate current drawn by the left hand or A sec- 
tion of V-401 during its conducting portion of the cycle. The accompanying increas- 
ed drop of plate potential of V-401A drives the grid of V-401B further negative so 
that a longer time is required for its potential to drift back to cut-off. The greater 
DC voltage level of the cathodes also increases the length of the self-blanking gate 
by increasing the required voltage range through which the grid of V-401B must 
pass before getting back to cut-off potential after being driven negative. The gradual 
increase in the self-blanking gate from its normal 175 microseconds to a maximum 


NegotK/e Trigger 
Pulse from 
Discriminator 



Fig. 2-17 Schematic diagram of coder 


Beacons and Interrogators 


2.17 


of about 1800 microseconds, lengthens the time that the coder is insensitive to fur- 
ther signals, thereby limiting the average rate of response to interrogations. 


175 - 1800 ^sec. 


BLANKING GATE 


CODE SPACING 


15 MICROSECONDS 
35 MICROSECONDS 


Transmitter Components 

The output voltage pips from the 
coder must be suitably shaped and ampli- 
fied for use in pulsing the magnetron. The 
circuits necessary to accomplish this are 
shown schematically in Figure 2-19. Each 
output voltage pip from the coder is ampli- 
fied and sharpened in V-610 which triggers 
the blocking-oscillator V-611. A tuned cir- 
cuit (L-601, C-608 and C-611) connected to 
the grid of the blocking- oscillator is used 
to control its period of oscillation. Regen- 
erative action, followed a half- cycle later 
by degenerative action, takes place in the 
blocking- oscillator. The oscillatory cir- 
cuit is tuned to about one mcps and makes 
one oscillation of large amplitude followed 
by a few highly damped oscillations. The 
output of the blocking- oscillator therefore 
consists of a positive pulse of one- half 
microsecond duration. This pulse is applied 
to the grids of the driver tube V-612 in which 
the pulse is still further amplified, and its 
top is flattened by limiting action in the grid 
circuit when grid current flows. The modu- 
lator tetrodes V- 901 and V-902 are normal- 
ly biased beyond cut-off. The pulse output 
from the driver stage causes the tetrodes 
to conduct for a period of one- half micro- 
second during which the plate potential of 
the modulators V-901 and V-902 is lowered from 15,000 to about 4000 volts. The 
high-voltage capacitor C-904 has previously been charged to a potential difference 
of 15,000 volts during the longer non-conducting period of the modulators. During 
the one half microsecond conducting period of the modulators, the lowering of the 
positive terminal of the high voltage capacitor from 15,000 to 4000 volts above 
ground results in the application of an 11,000 volt negative pulse to the cathode of 



^0 


_iUOL 


S L s 

Fig. 2-18 Formation of a typical 
code 



Fig. 2-19 Schematic circuit diagram of transmitter components 



2.18 


Beacons and Interrogators 


the magnetron the plate of which is at ground potential. During this time, the magne- 
tron oscillates and radiates energy into the wave guide leading to the transmitting 
antenna. 

Part in Brief Descriptions of Common Beacons and Interrogators 


The remainder of this section contains very brief descriptions of some of 
the better known radar responder beacons and interrogator- responser units. The 
material has been taken mainly from section 4 of the U. S. Badar Survey. 

A. Interrogator-Responsers 

Lucero is a British airborne interrogator^ of high power, operating at fre- 
quencies between 171 and 238 mcps. It is used for the following purposes: (1) Hom- 
ing onto long-range responder beacons; (2) Interrogation of IFF transponders on 
other aircraft; (3) Execution of rooster operations; (4) Beacon approach; (5) Hom- 
ing at medium range to light transportable beacons ; (6) Position-finding at medium 
ranges with Rebecca- H equipment. 

Rooster operation consists of calling for support with a signal upon which 
other friendly aircraft can home. For example, a reconnaisance aircraft which 
may have located an enemy objective, hovers over the target with its rooster bea- 
con turned on so that friendly bombers may reach the objective by homing on the 
beacon signal. 

Rebecca is a low- power British airborne interrogator designed to operate 
with Eureka ground beacons at spot frequencies between 215 mcps and 235 mcps. 
It is used primarily for short-range homing operations. Range and homing indi- 
cations are provided on an L-scan CRO. 

AN/APN-2, 2A, 2Y is an airborne 135-cm. interrogator- responser. It pro- 
vides range and relative bearing information for homing and navigational use. It 
is a modification of the SCR- 729 to perform the function of the British Rebecca 
Mark H. Range measurements may be made to within 200 yards. The maximum 
usable range varies from 25 to 100 miles depending upon the type of responder 
beacon interrogated. The AN/APN-2A model may act either as an interrogator- 
responser or as a transponder. 

B. Responders and Transponders 

Eureka is a British low-power ultra- portable responder beacon operating 
at spot frequencies between 215 mcps and 235 mcps and designed for use with the 
British airborne interrogator Rebecca. It is used primarily for short-range hom- 
ing operations and has a range of about 20-40 miles , depending upon the altitude 
of the interrogating aircraft. 

BOX (AN/CPN-6) is a ground-based or shipborne X-band range-coded 
responder beacon. It supplies the interrogating aircraft with range, azimuth, 
and identification information for navigation and homing. Its useful range is 
line-of-sight. 

BPS (AN/CPN-8) (an up-to-date version of BGS) is an air-transportable 
coded responder beacon which provides range, azimuth and identification for the 
guidance ^ aircraft equipped with S-band radar sets. Its maximum range is of 
the order of 100 miles. 


Beacons and Interrogators 


2.19 


Rosebud (AN/APN-19) is anS-band, range-coded beacon designed for use 
in aircraft. Installed in fighter aircraft, it enables GCI, SCI, MEW, or other 
radars to identify and vector such fighters at ranges greater than their detection 
ranges on enemy bombers. In fighters and bombers. Rosebud greatly increases 
the range and reliability of close-support bombing and photo- reconnaissance with 
SCR-584 radars equipped with plotting boards. The range is line-of-sight which 
corresponds to a maximum of about 250 miles at an altitude of 30,000 feet. 

Aspen (AN/APA-9) is an airborne S-band beacon designed especially for 
the Oboe Mark II navigational system. The reliable range is to the horizon (approx- 
imately 250 miles from an altitude of 30,000 feet.) 

BABS (AN/CPN-7) is a blind- approach beacon operating at a frequency of 
173.5 mcps. The ground installation is designed for interrogation by aircraft 
equipped with either SCR- 521 or SCR- 729 to provide for instrument approach to 
a landing field. The airborne equipment provides range and homing information 
on an L-scan CRO. The aircraft is guided to within one mile of the runway at an 
altitude of 200 feet and the actual landing is accomplished visually. 

Airborne Bups or Rosebups (AN/APN-29) is an S-band ultra- lightweight 
coded responder beacon designed for aircraft installation. Its range is line- of 
sight against a powerful radar. 

BPP (AN/PPN-2) is a lightweight paratroop responder beacon for use with 
supply aircraft equipped with Rebecca (AN/APN-2) interrogators. The beacon 
operates in the 135-cm. wavelength region and has a range of at least 40 miles for 
aircraft at a height of 5000 feet. 

AN/PPN-1 is a 135-cm. paratroop responder beacon for use with Rebecca 
interrogator- responser radars. It is designed especially for use with AN/APN-2, 
and is a close copy of the British Eureka Mark in. With AN/APN-2 at an altitude 
of 500 feet its range is about 18 miles with a 10 foot antenna height, and about 25 
miles with a 50 foot antenna height. 

YH or YH-1 is a 176 mcps land or shipborne beacon of use as a navigational 
aid. When interrogated by ASV, ASVC, ASE, AN/APX-2, and SCR-729 it responds 
with a coded reply on 177.5 mcps. It has a maximum range of the order of 100 
miles. 


YJ, YJ-1, YJ-2 are ground or ship-based responder beacons used as navi- 
gational aids to aircraft equipped with 176 mcps or 515 mcps search radar. The 
beacon replies with a gap-coded signal on either 177.5 mcps or 520 mcps depend- 
ing upon the frequency of the interrogating pulse. The useful range extends to about 
100 miles. 

AN/CPN-3 is an S-band range- coded ground responder beacon designed to 
provide homing and navigational aid to aircraft equipped with SCR- 517, -520, -717, 
-720, or AN/ APS- 2. The useful range is line-of-sight. 

BLACK MARIA is special identification equipment designed for installation 
in aircraft. It is triggered only by the simultaneous reception of pulses in the S 
and G bands, and it responds in the G band. 

BUPS (DC) (AN/UPN-l)isanS-bancl,battery-operated, self contained, ultra- 
portable, coded, responder beacon. Its reliable range is about 35 miles with SCR- 
717 or AN/APS-2 at 5000 feet. 


2.20 


Beacons and Interrogators 


BUPS (AC) (AN/UPN-2) is similar to AN/UPN-1 but is AC operated. 

BUPX (AC) (AN/UPN-3) IsanX-band, AC-operated ultraportable responder 
beacon. Its range is approximately 40 miles with AN/APS-3 or -4, and greater 
than 100 miles with AN/APS-10 or -30. 

BUPX (DC) (AN/UPN-4) is similar to AN/UPN-3 except that it is battery 
operated. 

AN/CPN-13, AN/CPN-15, and AN/PPN-8 are Mark V IFF transponder bea- 
cons. 


Beacons and Interrogators 


2.21 


Bibliography 



/ 

\ 

Identification 

Classification 

Title 

Issued by 

Chapter XHI 

Confidential 

Principles of Radar (a book) 

The Staff of the 

MIT Radar School 

590 

Confidential 

Siting and Range of Microwave 
Beacons 

MIT Rad. Lab. 


Secret 

Airborne Beacons for Aiding 
Control of Aircraft by Ground 
Radar — First Preliminary 
Report of Special Committee 
on Beacons 

General 

H.M.McClelland 

C ommunications 
Officer Army Air 
Forces 


Secret 

Ground Beacons for Air- Ground General 

C ooperation S econd Preliminary H .M .McC lelland 
Report of Special Committee on Communications 
Beacons Officer Army Air 

Forces 

Ship 290 

Confidential 

Preliminary Instruction Book 
for Radar Equipment AN/CPN- 
6 

US. Navy Dept. 
Bureau of Ships 

602 

Confidential 

The Statistics of Beacon In- 
terrogation 

MIT Rad. Lab. 

Section 4 

Navigational 

Radar 

Secret 

U. S. Radar Survey 

Div. 14 NDRC 

Section 2 
Airborne Radar 

Secret 

U. S. Radar Survey 

Div. 14 NDRC 

WA-3753.11 

Confidential 

LUCERO, T. R. 3566 

Air Ministry 

JEIA 3233 

Secret 

Rebecca and Eureka Equipment 
(Australian) 

Council for Scien- 
tific and Industrial 
Research, Radio 
Physics Laboratory 

91-3/31/44 

Confidential 

Video Stretching as a Method 
of Improving X-band Beacon 
Reception 

MTT Rad. Lab. 



Oboe 


3.01 


Type of System 
Range. 

Useful Range 

About 250 miles from 30,000 feet altitude. 

Accuracy and Precision 

The line of position of the aircraft is known to within about + 25 yards. 

Presentation of Data 

Aural on the controlled aircraft. 

Visual on PPI at the ground stations. 

Operating Skills Required 

Trained operators for aircraft. 

Trained operators for ground stations. 

Equipment Required 

300 lbs. of airborne equipment (AN/APA-9). 

Large heavy ground installations (modif ied SCR- 584 or British Oboe Mark II) . 

RF Spectrum Allotments Required 

Frequency 3150 to 3240 mcps. 

Bandwidth = 8 mcps . 

Present Status 

Operational. 

Oboe is an H or range type of system used primarily for precision blind 
bombing and photo reconaissance operations. The fundamental principles of opera- 
tion may best be described with the aid of Figure 3-01. and G 2 are suitably posi- 
tioned ground- stations each of which measures the range between it and the aircraft 
by pulse interrogation of a responder beacon in the aircraft. Station G^ supplies the 
aircraft with sufficient information to enable the pilot to fly a circular course at con- 
stant radial distance from G^. The radial range from Gj is chosen so that the arc 
of flight passes through a preselected target. Station G 2 measures the ground speed 
of the bomber along the arc, and from this speed and a pre-knowledge of the aircraft 
altitude and type of bomb used, transmits a bomb release signal to the aircraft at 
the instant that it reaches the proper range. 

The ground stations GjL ^2 are of the order of 100 miles apart, and control 
of the aircraft may take place at long ranges of 100 to 150 miles or more from the 
ground stations. 

Various names are used to designate the stations Gi and G 2 respectively such 
as: cat and mouse stations; tracking and release stations; or drift and rate stations. 

Both cat and mouse stations transmit on the same radio frequency, but use 
different pulse repetition rates. The necessary signals to the pilot to keep the air- 
craft on course and to the bombardier to indicate the desired instant of bomb release 
are transmitted by means of either space or width modulation of the same pulses 
that are used for range measurement. The tracking signals from the cat station to 
the pilot of the aircraft consist of aural indications of the dot- dash type. A steady 
tone of moderate intensity is used for the "on course" indication. A series of dots 
or a series of dashes is heard if the bombing aircraft is off course to the right or 
to the left respectively. The intensity of both dots and dashes gradually increases 


3.02 


Oboe 



Gg (MOIBE) 


FRIENDLY 

TERRITORY 


bomb RELEASE POINT 


Fig. 3-01 Geometry of approach to the target in Oboe bombing system 

The cat station transmits the tracking intelligence. 

The mouse station G 2 transmits the bomb-release signal. 

as the aircraft deviates further and further from its proper course until at a distance 
of about 200 yards off course a maximum tone is reached corresponding to 100%am- 
plitude modulation; after which there is no further increase in the volume of dots or 
dashes with further deviation from the course. 

The course information may be obtained by either space or width modulation 
of the pulses transmitted from the cat station. The space modulation scheme is 
illustrated by Figure 3-02. Every other transmitted pulse of the 266 or 194 pulses 
per second is fixed in time phase. The relative position of the intervening pulses 
can be varied so that they occur at any desired point between 1/2 and 3/4 of the time 
spacing between the fixed pulses. The normal "on course” position of the movable 
pulses is 5/8 of this distance. When in the 5/8 position, the energy in the movable 
pulses neither adds to nor subtracts from that contributed to the tuned filter by the 
fixed pulses. When the movable pulses are halfway between the fixed pulses, their 
energy adds to that of the fixed pulses and the response of the tuned filter in the 
receiver is a maximum; and when the movable pulses are located at three-quarters 


Oboe 


3.03 



oott at 

100% Modulation 


Dots ot 

50% Modu lotion 


ooshos at 
100% Modulation 


Dashes at 
50% Modulation 


zero VioJulotion 
"on course " 


Tone Intensity 

F/W/Z[W//2 





I 


I 


Fig. 3-02 Space or phase modulation of the ground- station signal pulses 


of the way between the fixed pulses, the effects of the fixed and variable pulses cancel 
one another resulting in zero output sigpial from the tuned filter. 

Whenever the aircraft is flying at the proper range from the cat station, 
the variable pulses are always in the 5/ 8 position resulting in a steady output tone 
of moderate intensity. If the aircraft deviates from the proper range, the phase of 
the variable pulses from the cat station is automatically keyed back and forth be- 
tween certain limits (such as A and B in Figure 3-02) equidistant on either side of 
the 5/8 position. Within the 1/2 and 3/4 position limits, the amoimt of phase shift 
back and forth from the 5/8 position increases the further the aircraft is from its 
proper course, but the time spent in each position is not the same, and whichever 
time is the greater depends upon whether the range error is positive or negative. 


3.04 


Oboe 


For example, if the range of the aircraft from the cat station is slightly too small, 
the phase or position of the variable pulses might shift back and forth between the 
limits A and B of Figure 3-02, remaining in position A only long enough to allow for 
the transmission of a dot signal of increased intensity, and then shifting to position 
B for a somewhat longer period during which the tone intensity is reduced from 
that of the "on course" indication. If on the other hand, the range of the aircraft 
from the cat station is slightly too large, then the phase timing of the variable 
pulses transmitted from the cat station would be such that the phase corresponds 
to position A for a longer period than for position B so that the output signal from 
the tuned filter consists of dashes of increased intensity with shorter spaces of 
lesser intensity. Thus it is seen that either dots or dashes may be formed by shift- 
ing the movable pulses back and forth between limits corresponding to the depth of 
modulation, with the relative lengths of time during which the pulses remain in each 
position determining whether the output of the filter will consist of dots or dashes. 

In the width- modulated system, all pulses transmitted from either the cat 
or mouse stations are fixed in their space or phase relationship, but vary in width 
in accordance with the modulation. Circuits in the aircraft receiver are so arrang- 
ed that the intensity of the response of the tuned filter varies from zero to a maxi- 
mum as the width of the received pulses varies from one to three microseconds 
duration. The "on course" signal is produced when all pulses have a width of two 
microseconds. Relative intensity modulation of the desired depth may be obtained 
by shifting the pulse width (within 1-3 microsecond range) back and forth between 
values equidistant above and below the "on course" value of two microseconds. By 
properly controlling the relative periods during which the pulse width is set at one 
or the other of the two limits, desired dot or dash signals may be produced in the 
same manner as in the space or phase modulation system previously described. 

The airborne part of the Oboe equipment (Aspen) consists of the antenna sys- 
tem, including the RF plumbing and controls for directing the antennas towards the 
cat and mouse stations; a receiver; a repetition rate filter unit for obtaining audio 
signals for the pilot and bombadier; a modulator which includes the transmitting 
equipment and in which is also located the T-R box, the local oscillator and crystal 
mixer of the receiver, and a pre- amplifying unit which operates at the intermediate 
frequency of the receiver; a control- junction box which contains the controls for the 
system; and a power supply. 


The operation of the Aspen unit of the Oboe system is illustrated by the block 



Fig. 3-03 Block diagram of the airborne Oboe equipment AN/APA-9 (Aspen) 


Oboe 


3.05 


diagram of Figure 3-03. An incoming RF signal pulse is received by the antenna 
and passes thro\igh a T-R box, into a crystal mixer. The resulting intermediate- 
frequency signal passes through a pre-amplifier and then to the receiver proper. 
The receiver is either a British "Penwiper" receiver or a British "Pepperbox" re- 
ceiver. It has two outputs, one of which is a trigger which actuates the modulator 
and causes an RF beacon response pulse to be transmitted for each received signal 
pulse. The other output from the receiver is fed to a British filter box, which con- 
tains two peaked au^o amplifiers one of which is resonant at the characteristic 
pulse repetition rate of the cat ground station, and the other of which is resonant at 
the repetition rate of the mouse ground station. The outputs of the filter unit are 
connected to the headphones of the pilot and bombadier so that the appropriate aural 
signals are conveyed to them for control of the aircraft course and to indicate the 
moment of bomb release. The amplifiers in the receiving system are made insensi- 
tive for a short interval during the transmission of the beacon response pulse. This 
gating function is indicated by lines from the control box in the block diagram of 
Figure 3-03. 

The microwave (f = 3150 to 3240 mcps) Oboe groxmd stations used to track 
the aircraft are either Oboe Mark II, which is a modification of the original British 
Mark I system, or Oboe Mark II HSM which is a modified SCR- 584. The British 
systems use the Mark I console, ASG modulator and RF head, and a modified anten- 
na. 


The basic geodetic data supplied to the groimd stations for an Oboe operation 
consists of the ranges from the tracking and releasing stations to an aircraft at a 
pre-selected height directly above the target. A number of corrections must be 
applied in order to determine the correct range for bomb release. At long ranges 
where the curvature of the track is slight, very little error is introduced by using 
the measured average ground speed, but at short ranges where the angle of cut 
changes considerably, appreciable error may be introduced into the calculations due 
to the change in the angle between course and wind directions as the aircraft flies 
around the circular track. The groimd speed would change even if the aircraft flew 
with constant "effort" but it flies with constant airspeed which further complicates 
the problem. The calculations necessary for the determination of the correct bomb- 
release range are discussed in AWAS note No. 16 entitled "Theory of the Average 
and Instantaneous Velocity Measuring Mouse". 

Bibliography 


Identification 

Classification 

Title 

Issued by 

M-148-C 

Confidential 

Handbook of Instructions for 
Radio Set AN/APA-9 

MIT Rad. Lab. 

Section 4 

Navigational 

Radar 

Secret 

U. S. Radar Survey (pages 

4-167 to 4-171) 

Div. 14 NDRC 

63-9/16/43 

Confidential 

Comparison of Vector and Dot- 
Dash Methods in the Oboe 
Steering Problem 

MIT Radj Lab. 

Note No. 16 
August 1943 

Secret 

Theory of the Average and 
Instantaneous Velocity 
Measuring Mouse 

A.W.A.S. 



SHORAN 


Type of system 

Range or distance (H system). 


4.01 


I 

\ 


Frequency and Wavelength 

210 - 320 mcps (1.43 - 0.94 meter). Bandwidth: 4 mcps. 

Useful range 

Line of sight, 180 miles at 12,000 feet, (depends on height of craft) 

Accuracy 

Estimated probable precision of fix (theoretical) i 50 feet. 

Equipment required 

(a) Ground: Two beacon responders, transportable by truck. Directional 
antennas used, which may be mounted on 50 ft. masts. Careful location of beacons 
required to give estimated accuracy noted, (b) Craft: (AN/ APN3) Interrogating trans- 
mitter, receiver and highly specialized timing and indicating circuits. Total weight: 
232 lbs. 

Operating Skill Required ' 

(a) Ground beacons will run unattended but skilled crew is needed for tri- 
angulation of site and setting up of beacons, (b) Navigator with some special train- 
ing required in craft. 

Presentation 

Pip matching on a 3- inch CRO. When this has been accomplished, Shoran 
distances from ground beacons are read directly on mileage dials. 

Present status 

Developed by RCA and extensively used in the latter part of 1944 and early 
part of 1945 in Europe as a precision bombing device. In this capacity, Shoran has 
given extremely accurate results. 


General Features 

The Shoran system was developed primarily as a precision blind bombing 
device, at relatively short ranges (100-200 miles). As with any device which can 
be used to navigate a craft to a specific point, Shoran can also be used as a precision 
navigational aid. However, it is not specifically adapted to flying a predetermined 
but arbitrarily selected course, and as a long-range navigational aid would be of no 
use in its present form. Accuracy was a first consideration in the development of 
the existing equipment, simplicity and man-power requirements being deliberately 
sacrificed. In use as a precision bombing device, a computer is added to the air- 
borne equipment. Since this report is concerned with navigation, the action of the 
computer is not discussed. 


Principles of the System 

The two ground beacons are located in suitable positions (high ground). The 
accuracy of the system depends on the accuracy with which the beacon positions are 
known. The craft carries a transmitter (3 watts average power) which radiates 

pulses (i microsecond duration) in alternate groups at two different frequencies . 

z 


The beacon receivers are tuned to these two frequencies, so that both beacons are 
interrogated by the craft transmitter, but independently. The reception of a pulse 
from the craft by a beacon causes the beacon to re-radiate a pulse (on a different 
frequency) which travels back to the craft and is there received and displayed. Means 
are available in the craft for measuring accurately the time taken for the pulses to 


4.02 


SHORAN 


travel from craft to beacon and back. The two time intervals so determined (one 
for each beacon) enable the craft to determine its distance from each of the two bea- 
cons. This yields a fix, as the point of intersection of the two circles whose centers 
are the two beacons and whose radii have been determined. 

The two beacons are known as the "rate" and "drift" stations. To reach a 
predetermined point, whose Shoran distances from the rate and drift stations are 
known, the procedure is as follows. The craft will navigate (by Shoran or other 
means) until it is proceeding along an arc whose center is the drift station and whose 
radius is the required drift station distance from the destination. Proceeding along 
this arc, the distance from the rate station changes progressively. When this dis- 
tance is equal to the required rate station distance from the destination (as observed 
by the navigator) the designated point has been reached. 



Fig. 4-01 Ambiguities 


SHORAN 


4.03 


Referring to Figure 4-01, suppose that the destination point A is 130 miles 
from beacon P and 110 miles from beacon Q. If Q is to be the drift station, the 
approach may be as shown by arrows. Clearly the approach might have been from 
the opposite direction, and equally P might have been chosen as the drift station so 
that there are four possible lines of approach using the drift- and- rate procedure. 
The advantage of this procedure is that one of the distances being observed (the drift) 
remains constant during the final approach, leaving the navigator free to concentrate 
on the rate reading. 

As in all systems of this type, there is ambiguity as between points A and B^, 
both of which lie at the required distances from the beacons. It is assumed that the 
two sets of beacon pulses may be identified at the craft, so that the craft will not 
arrive at a point 130 miles from Q and 110 miles from P. Other ambiguities exist 
however, due to the fact that the Shoran indicator indicates tens and units of miles 
(as well as tenths and hundredths) but not hundreds of miles. Thus the Shoran indi- 
cator will also give the required indications at points B 2 (30,110; 130,10; 230,110; 
230,210 etc.) 

Shoran Distance 

Distinction must be made between geographical (great circle) distances and 
Shoran distances. 



Due to refraction, radio waves do not travel in straight lines. The assump- 
tion usually made is that above a certain height the actual path is an arc of about 
15,000 miles radius. Furthermore the actual velocity of radio waves will not be 
constant along the Shoran path, neither will the heights of beacon and craft above 
sea level be the same in general. For these reasons, the Shoran distance (S, Fig- 
ure 4-02) will be greater than the great-circle distance M by an amount A, so that 
S = M + A. The correction A is given to a close approximation by the formula 

A = 2.152. M.(H + K) + 1.794 . (H - - 0.2477 

10® 10® M 10® 

where the symbols have the meanings indicated in Figure 4-02 and H and K are 
measured in feet, A and M in miles (statute). 

Regarding the theoretical precision of Shoran, RCA gives the following table 
of causes of error and their estimated contributions: 


4.04 


S HORAN 


Source of Error Maximum Estimated Error 


(1) 

Residual approximation errors in the 
computed corrections 

± 20 ft. 

(2) 

Drift of craft timing frequency after 
checking with ground station 

± 30 ft. 

(3) 

Setting and reading mileage dials 

± 10 ft. 

(4) 

Scale non-linearity in craft timing 
phase- shift circuits 

± 60 ft. 


Maximum estimated possible error 

± 120 ft. 

Since it is unlikely that all four of the component errors will be simultaneous- 
ly of the same sign, the estimated probable error is given as + 50 ft. These errors 


refer to the Shoran distances of the craft from the beacons. Thus there will be an 
area of uncertainty determined by arcs of position representing the limits of rate and 
drift radii. This area of uncertainty will have a minimum value if the rate and drift 
circles intersect at right angles. (See Section 1). 

Principles of Operation of the Equipment 

Referring to the block diagram of the equipment carried in the craft (Figure 
4-03), there is a commutator which is motor-driven from the main power source 
(d-c supply in the case of aircraft). This commutator performs a complete sequence 
of switching operations every 1/10 second. The craft transmitter (40) is pulsed by 
the timing gear at a pulse repetition rate of approximately 930 cps. Since the craft 
transmitter must interrogate two beacon transponders on different frequencies, and 
since the interrogating pulses for the rate beacon must be phased differently from 
those which are to interrogate the drift beacon, two pulse outputs are provided from 
the timing gear. These two pulse outputs are used alternately for periods of 1/40 
second with idle periods of 1/40 second interspersed between them. This is one of 
the functions of the commutator. In synchronism with this operation, the radio fre- 
quency of the transmitter must be switched to coincide alternately with the frequencies 
assigned to the two beacon receivers. This is accomplished by having another sec- 
tion of the commutator operate a relay which short-circuits a portion of the trans- 
mission-line section which determines the transmitter oscillator frequency. This 
sequence of operations is represented in Figure 4-04. It will be seen that the plane 
transmitter frequency is shifted by an amount Af during portions of the cycle. This 
amount is of the order of 15 - 30 mcps. 

The frequency of the crystal oscillator is actually 93,109 cps. (93.109 kcps). 

In the block diagram and in the discussion which follows, this figure has been rounded 
off to 93 kcps for the sake of simplicity. The reasons for the selection of this basic 
frequency (identical in all Shoran timing circuits) is as follows: 

(1) Consider a craft - beacon distance of 100 miles. The total distance to be covered 
by the pulse which interrogates the beacon and by the response transmitted back 
from beacon to craft will then be 200 miles and the time taken for its round trip 
is 1074^ sec. Since it is desirable to allow time for a response to be received 
due to each transmitted pulse before the emission of the next pulse, this means 
that the pulse repetition rate (prf) must not be greater than 931.09 pps (corres- 
ponding to a period of 1074 ^ sec.) if 100 miles is the maximum range to be indi- 
cated. 

(2) Since the final indication is to be by means of pip alignment on a circular CRO 


SHORAN 


4.05 




I 

\ 






4.06 


SHORAN 



time (sec.) 

Fig. 4-04 Transmission sequence 

A: plane transmitter interrogates beacon P on frequency f 
B: plane transmitter interrogates beacon Q on frequency f + Af 
C: inactive periods 


sweep, the frequency of the sweep should not be higher than 931.09 cps, in line 
with the above considerations. 

(3) Accuracy demands that much higher sweep speeds should be available for deter- 
mination of miles and fractions of miles. In the present case, sweep speeds of 
9,310.9 cps (corresponding to a 10 mile range) and93,109 cps (corresponding to 
a 1 mile range) are provided. 

(4) A lower prf would allow ranges of over 100 miles to be indicated directly; but 
since the operational range of Shoran is limited by propagation considerations 
to something of the order of 200 - 250 miles, and since the operator is presumed 
to have other information which will enable him to supply the number of hundreds 
of miles, it is apparent that the extra complication introduced by a fourth sweep 
speed would not be justified. 

(5) The prf should be as high as possible consistent with (1) above, in order to enable 
the maximum amount of intelligence to be transmitted in a given time. Further- 
more, a prf of about 930 pps means that about 23 pulses will be transmitted in 
each 1/40 sec. period during which the transmitter is pulsed. It would not be 
desirable further to reduce this number. 


In line with the above considerations, the crystal oscillator (1) is followed by 
two frequency dividers (3) and (4) each of which divides by 10. These are of the re- 
generative type, whose action may be explained by reference to Figure 4-05. The 
input signal, of frequency 93 kcps (f), is applied by way of to the control grid of 
the mixer tube V 2 . The plate circuit of V 2 is tuned to 0.1 f (9.3 kcps). Assuming 



Fig. 4-05 Frequency divider 



SHORAN 


4.07 


a signal to exist at this frequency, this signal is fed back to the control-grid of the 
multiplier tube V^, which is driven sufficiently hard to operate in a non-linear man- 
ner. The plate circuit of is tuned to 0.9 f, and the resulting signal at this fre- 
quency is applied to the grid of V 2 together with the original input of frequency f. 
These two signals, beating in the non-linear mixer tube V 2 , provide the necessary 
output at 1/10 f. This system may be thought of as a regenerative, non-linear amp- 
lifier with a tuned output, which is not self-sustaining. The phase of the output is 
stable and is correlated with that of the input. 

Referring again to the block diagram (Figure 4-03) the oscillator (1) and fre- 
quency dividers (3) and (4) furnish signals of frequencies approximately 93 kcps, 
9.3 kcps and 0.93 kcps. These three signals are applied to three suitably designed 
quadrature networks (5), (6), (7). Each quadrature network gives two outputs which 
are in quadrature (90® phase relationship) with respect to each other. These quad- 
rature outputs are used for two general purposes: (a) generation of a circular sweep 
on the cathode- ray tube indicator, (b) generation of suitably phased marker pulses 
and transmitter pulses. The selector switch (8) (which is a part of the multi-gang 
range switch) places the selected pair of quadrature voltages on the horizontal and 
vertical deflection plates of the cathode- ray tube (9), thus yielding a circular sweep 
whose frequency is (approximately) 0.93-kcps on the 100 mile range, 9.3 kcps on the 
10 mile range and 93-kcps on the 1 mile range. 

There are three main functions performed by the remaining components of 
the timing circuits: 

(a) Generation of pulses used to trigger the transmitter in the craft. As previously 
explained, the frequency of these pulses is 930 pps (on all ranges) and two sets 
of pulses are required to be available, with different phasing, one set used dur- 
ingperiods A (Figure 4-04) when transmission is at the frequency to which the P 
beacon responds , and the other set used during periods B (Figure 4-04) when the 
frequency is changed to that to which the Q beacon responds . 

(b) Generation of marker pulses. These serve as a fixed time reference: they ap- 
pear as an outward deflection at the top of the circular sweep, and the received 
pulses from the beacon transponders are to be aligned with the marker pulses. 

(c) Generation of suitable blanking and intensifying pulses. 

If the marker pulses are to be fixed on the circular trace, and if the received 
pulses are to be aligned with them, it follows that the transmitted pulses must be 
advanced in phase with respect to the marker pulses by an amount whose correspond- 
ing time- advance is exactly equal to the time of transit of the interrogating and reply 
signals plus the delay time associated with the beacon. The latter time is standard- 
ized at 1.93psec. corresponding to an extra distance of 0.18 mile for all Shoran 
beacons. This phase advance is performed by the calibrated variable phase shifters 
(10) through (15). These phase shifters are of the continuous type, in which the two 
quadrature inputs are applied to two stator coils oriented so that their planes inter- 
sect at an angle of 90®, and the output is taken from a rotor coil, the angle of which 
(with respect to the stator coils) determines the relative phase of the output. Con- 
siderable care was taken in the design of these phase shifters to make them as near- 
ly linear as possible, that is, the phase-shift obtained is very nearly proportional to 
the angle through which the rotor is turned. These components are among the most 
critical in the system: the indicating dials of the phase shifters are calibrated dir- 
ectly in miles, and the accuracy attainable depends on the precision with which the 
phase shifters can be constructed. The six phase shifters are ganged in two groups 
of three each: one set is concerned with the phasing of pulses for the rate station 
and the other for the drift station. Two '’pulse selectors" (29) and (30) generate 
pulses for the craft transmitter. Each pulse selector receives three inputs: 93 kcps 


4.08 


SHORAN 


sinusoidal, 9. 3 kcps pulses (of width about lljLi sec) andO.93 kcps pulses (width about 110 
;jsec). These input pulses are produced by conventional clipping, differentiating and 
clipping circuits at (20) through (23). The action of the pulse selectors may be ex- 
plained by reference to figures 4-06 and 4-07. The three inputs are applied to three 
grids of a multi- grid tube (Figure 4-06). Biasing potentials are so arranged that plate 
current will only flow if all three grids are simultaneously gated positively. 


Considering now Figure 4-07, it is seen that only once in each period of 1074 
;isec. (corresponding to a frequency of 0.93 kcps) will the required condition exist 
and plate current flow. The output at the plate of the tube will therefore consist of 
pulses at a repetition rate of 930 pps. The width of the pulses is about 2 ^sec. 


It will be observed that the exact position in time of these pulses depends on 
the phasing of the three inputs. One complete rotation of the 93-kcps phase shifter 
(360® phase shift) will shift the pulses 10.74 jj sec. corresponding to a change in bea- 
con-craft distance of 1 mile. Similarly one complete rotation of the 9.3-kcps phase 
shifter will produce a time-shift of 107.4 yusec. (10 miles), and a complete revolu- 
tion of the 0. 93-kcps phase shifter will give a time-shift of 1074 ;jsec (100 miles). 
However, a given change in one phase shifter must be accompanied by a proportional 
change in each of the others, in order for the required time-coincidence between the 
waveforms of Figure 4-07 to be main- 
tained. For this reason, the three 
phase shifters of each set are geared 
together. Twenty revolutions of the 
handwheel on the front panel produce 
one revolution of the 93-kcps phase 
shifter simultaneously with one- tenth 
of a revolution of the 9.3-kcps phase 
shifter and one one- hundredth of a re- 
volution of the 0. 93-kcps phase shift- 
er. This process may be thought of 
as constituting a movement of the 
whole of Figure 4-07 to right (or 



Fig. 4-06 Pulse selector 


f 1 


93 kc/s 



— *1 no r 

iiOpsec. pulses 


9.3 kc/s 


0.93 k9/s 


2>jsec pulses 
-1073,2 psec.- 


output 


0 100 200 300 


►}jsec. 


Fig. 4-07 Pulse selection 


SHORAN 


4.09 


left) by 10.74 jj sec with respect to some arbitrary (but fixed) time scale. The number 
of rotations of the various phase shifters is recorded in units, tenths and hundredths 
of miles by suitably geared counters which therefore give direct range indications 
(one for the rate station and one for the drift station) on the front panel. Tens of 
miles are indicated on a dial mechanically connected to the 0.93-kcps phase shifter. 
As previously noted, the number of hundreds of miles must be known by other means. 
In order to avoid the very large amount of cranking that would be necessary for a 
shift of (say) 30 miles, provision is made for the 0.93-kcps phase shifter to be dis- 
connected mechanically from the rest of the gear train and reset in any one of ten 
preset positions, any of which yields correct phasing and gives movements of multi- 
ples of 10 miles. Since the counters indicate nothing larger than ten miles, their in- 
dication is not upset by this operation. 

(36) represents a part of the motor- driven commutator which performs the 
switching operations indicated in Figure 4-04, together with (43) which effects the 
change in the transmitter frequency. (42) is a push-button switch (normally closed) 
which normally allows the transmitter pulses to be used for blanking the CRO beam so 
that actual transmitted pulses (which may come through the receiver circuits even 
though receiver and transmitter are not tuned to the same frequency) will not be dis- 
played on the CRO trace. To check the zero adjustment of the equipment, (42) is 
pressed. This allows the transmitter pulses to appear on the display. Since the 0.18 
mile beacon delay must be allowed for, the marker and transmitter pulses should be 
aligned when the phase shifters are set for a corresponding delay. Since the phase 
shifters produce an advance (instead of a delay) in the transmitted pulse, they must 
be set at -0.18 mile, or in other words at 99.82 miles, when this adjustment is to be 
made. The marker pulses are then moved slightly by means of (41) until coincidence 
between them and the transmitted pulses is obtained. 

Marker Pulse Ge ne ration and Blanking 

The following problems arise in connection with the display of marker and 
received pulses: 

(a) Marker pulses appear at a frequency of 0.93 kcps. On the 100-mile sweep, this 
results in one marker pulse per sweep, but on the 10- mile and 1-mile sweeps 
there will be one pulse every ten sweeps and one pulse every hundred sweeps 
respectively. The same considerations apply to the received pulses, which will 
have the same prf as the marker pulses. This would mean that the circular trace 
would be much brighter than the marker and received pulses when using the 10- 
mile and 1-mile sweeps. It is therefore desirable to blank out all sweeps except 
those during which the marker and received pulses occur and at the same time 
to intensify the desired sweeps. This function is known as "circle blanking" and 
is accomplished by pulse selectors (38) and (39). 

(b) It is necessary to blank the CRO beam at the instants when the craft transmitter 
is pulsed. This is accomplished as already explained in connection with switch 
(42), which is in the cathode circuit of the blanking- pulse amplifier (44). The 
blanking pulses are mixed and applied to the grid of the cathode- ray tube. 

(c) In order to distinguish between the received rate and drift pulses, the rate pulses 
are made to deflect the beam outward and the drift pulses inward. This is accomp- 
lished by (27) in conjunction with a section (28) of the commutator. 

(d) If the output of the receiver (19) remained connected at all times to the central 
deflecting electrode of the CRO (9), noise voltages would be displayed as clutter 
on all parts of the sweep. For maximum operational range, the amount and in- 
tensity of the clutter should be as small as possible. For this reason, it is ad- 
vantageous to disconnect the receiver output during the display of the marker 
pulses (see also (e) below). This is accomplished by suitable pulses taken from 
(34) and applied to (27) and is known as "receiver blanking". 


4.10 


S HORAN 


(e) If the marker pulses occur on the same sweep as do the receiver pulses and if 
the pulses are being aligned, then when all three pulses are nearly in alignment 
it will be difficult to distinguish one pulse from another. This arises from the 
fact that as the pulses start to overlap, they will add instead of being superposed. 
Figure 4-08 illustrates this for idealised pulses. 



Pulses Added Pulses superimposed 


Fig. 4-08 


This indicates that marker and received pulses should occur on different sweeps. 
Provided that the marker is accurately positioned on any sweep, it does not matter 
which sweep is used. On the 100-mile sweep, when a marker pulse occurs on 
every sweep, this "offsetting" of the marker pulse is not possible, but neither 
is it important, since the final pulse alignment will not be done on the 100- mile 
range. On the 10- mile and 1-mile sweeps, the marker pulse is offset by one and 
by ten sweeps respectively. This is accomplished by using a 0.93-kcps source which 
is phased differently from the regular 0.93-kcps source, for the generation of mark- 
er pulses in the 10-mile and 1-mile switch positions. This process is known as 
"marker offsetting". 

Considering now the circuits by means of which the above five problems are 
solved, the pulse selector (37) operates in the same way as previously explained for 
(29) and (30), and its output consists of 2 jusec. pulses with a prf of 0.93 kcps which 
are to be used as markers. (41) allows for small changes in phase of the 93-kcps 
input for the purpose of zero setting as already explained. The 0.93-kcps input is tak- 
en either from the "regular" phase setting or from the "offset" phase setting, depend- 
ing on the position of the range selector switch (31). The pulse selectors (38) and (39) 
have only two inputs (9.3 kcps and 0.93 kcps) and are only operative in certain switch 
positions. When operative, they function on the same principle as that described for 
the three-input pulse selectors, and deliver llO^j sec. pulses with a prf of 0.93 kcps, 
which are used to intensify the CRO beam (normally at threshold intensity) at appro- 
priate periods as noted below. The phase changers (16) (17) (18) and the pulse-shap- 
ing circuits (24) (25) (26) have the same functions as described in connection with the 
generation of transmitter pulses. 

Scrambling 

Since a number of craft may be interrogating the beacons during any given 
period, there is a possibility that spurious pulses may appear on the display. That 
is, craft A may see the pulses resulting from beacon interrogation by craft B, as 
well as those produced by its own interrogation of the beacons. In order to minimize 
this effect, there is on the commutator a section (represented at (2)) which renders 
the first frequency divider inoperative (by gating its suppressor grid) for two periods 
of 1/40 sec. during each 1/10 sec. cycle of switching operations. This is represented 
ate in Figure 4-04. Now under the least favorable conditions, all the crystal oscilla- 
tors in interrogating craft might have exactly the same frequency and all the spurious 
returns would therefore lock in stationary positions on all displays. But since the 
commutator is motor-driven from the craft' s power source, and since the probabi- 
lity of 20 such motors running at precisely the same speed is extremely remote, the 


SHORAN 


4.11 


sequence of dividing operations will not re-start in identical phase on all craft after 
each 1/40 sec. interruption. The spurious returns will therefore move at random all 
over the trace, and although contributing to clutter will not be confused with the craft's 
own return, which i^ locked in fixed position on the sweep. This function is known 
as "scrambling". A corresponding section of the commutator (35) ensures that the 
CRO beam is completely blanked while the scrambling switch is open, by cutting off 
the plate supply voltage to the blanking- pulse amplifier for appropriate intervals of 
time. 


Consider now the situations existing on each of the three range- switch positions . 

(a) 100- mile swee p: There are 930 rotations of the sweep per sec. Pulses are trans- 
mitted (and received) at a rate of 930 per sec. Every sweep is used and none 
are blanked, since the pulse selectors (38) and (39) receive no input signals . Mark- 
er pulses (regular) are generated by (37). The trace is blanked while the trans- 
mitter is operative. The brilliance of the display is controlled only by the intensity 
control on the CRO (not shown on the block diagram). 2000 rotations of the hand 
crank, causing 1 rotation of (14), 10 rotations of (12) and 100 rotations of (10), 
would cause the rate pulse to be shifted through one complete sweep (100 miles 
range change). 

(b) 10- mile swee p: There are now 9,300 revolutions of the sweep per second and 
930 marker pulses per second, also 930 beacon responses. The marker pulses 
are offset by a time corresponding to one sweep. Thus the beacon response, if 
visible at all, will occur on the sweep preceding that on which the marker pulse 
occurs. These two sweeps only are allowed to appear. The remaining 8 out of 
every 10 are blanked, since (38) and (39) now generate pulses which are used to 
intensify the beam during the two sweeps corresponding to marker and received 
pulses. Theoverallbrillianceof the display is thus unchanged. The beam is also 
blanked while the transmitter is operative. A given rotation of the mileage dials 
will now produce ten times as much movement of the received pulses around the 
trace as it did in (a). Due to the position of switch (34) radial deflection due to 
receiver signals is now prevented during the sweeps on which the (offset) marker 
pulses occur. 

It is to be noted that the received puls.es will not appear on the display at all 
unless they were approximately aligned with the marker pulse on the 100- mile 
sweep. 

The sequence of events now occurring is shown approximately to scale in 
Figure 4-09. In this diagram only one sequence of pulses is represented (rate 
or drift) and no attempt is made to show actual amplitudes or waveforms. Pulses 
which result in blanking or disabling are shown below the axis, and intensifying 
pulses above. It is assumed that the marker and received pulses have been 
correctly aligned, and that the approximate range shown on the dials is 30 miles. 

(c) 1-mile swee p: There are now 93,000 sweep revolutions per second, and in each 
100 of these there will occur one marker pulse and one received pulse. The 
marker pulses are offset by 10 miles = 10 sweeps. Supposing the marker and 
received pulses to be aligned, then if received pulses occur during the 1st, 101st, 

201st sweeps of a particular sequence, marker pulses will occur during 

the 11th, 111th, 211th sweeps. The beam is blanked while the transmitter 

is operative. A given rotation of the mileage dials will now produce one hundred 
times the displacement of the received pulse compared with (a) above. (39) now 
generates intensifying pulses of 11 |jsec. duration (= 1 sweep) and recurrence 
rate 930 pps., corresponding to the sweeps on which the (offset) marker pulses 
occur, and (38) performs the same function for the sweeps on which received 
pulses are liable to occur if the pulses have been previously aligned on the 100- 
mile and 10-mile range scales. Due to the position of (34), the receiver output 


4.12 


SHORAN 


sweeps 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 1 I i i 1 

Marker 

Pulses 

1 1 1 1 

Transmitted 

Pulses 

- 1 ( ( 

Received 

Pulses 

N \ \ 

1 \ \ \ 

1 \ \ \ 

i ■ 1 

Transmitted 

Pulse 

-J 30 L 
^ Miles , 

1 1 

Blanking 

Beam 

Intensified 

1 1 1 

1 — 1 1 — 1 1 1 1 1 

("circle 

Blanking") 

Receiver 


Blanking 

U Li U U — 


Fig. 4-09 Time relations 


Note: each sweep represents 107.4 microseconds, 
or 10 miles range change. 


is disconnected from the indicator for periods of 110 jjsec. of which the 11^ sec. 
sweep containing the marker pulse is the center portion. Operations controlled * 
by the commutator (scrambling, 1/40 sec. CRO blanking, pulse polarity switch- 
ing, transmitter frequency and pulse phase switching) proceed at all times, ir- 
respective of the position of the range switch. 

Polarity Switching and Disabling Circuit 

The functioning of the block denoted as (27) is illustrated by the circuit of 
Figure 4-10. Receiver pulses are applied to the control grids of both Tj and Tj at 
A. The screen supplies of the two tub^s are gated alternately by S which represents 
asectionof the commutator ((28) in Figure 4-03). Tj functions as a normal (invert- 
ing) amplifier andTg as a (non- inverting) cathode follower. The outputs of both tubes 
are placed in parallel by a 0.2 /jf condenser, so that both tubes work into the same 
load The amplitude of the output will be the same no matter which tube is on but 
will be inverted (with respect to the input) if Tj is on, non- inverted if Tg is on. The 

output IS taken at B. Negative receiver-blanking pulses (from (34) in Figure 4-03) 
are applied at C, thereby gating the suppressor grid of Tj or the screen grid of To 
depending on which tube is on. This prevents any receiver signals from appearing 
at the output terminal B during the sweep on which the (offset) marker pulse occurs. 


SHORAN 


4.13 



Crystal Frequency Check 

By means of a small variable shunting condenser, the frequency of the crystal 
oscillator (1) (Figure 4-03) may be adjusted to coincide with that of the crystal at 
one of the ground beacons, which is accurately stabilized. This operation is per- 
formed preferably immediately before a reading is taken. It will be realized that 
any departure of crystal frequency from its assigned value of 93.109 kcps represents 
a source of error in the system. When the frequency of the oscillator has been so 
adjusted, frequency- calibrating pulses transmitted from the ground stations and re- 
ceived at the craft will remain stationary on the display. 

The information here presented was obtained from various copies of the peri- 
odical "Radar", and from the handbook of maintenance instructions for Radio Set 
AN/APN3 (CO- AN 08-30 APN3-2-M). 





V . 


. 

* kfr .- 
a ^ t ', -* 


* w 

' ^^KiTjm *kix^ j» I , ‘ ‘2'* 


L » • » y 7 * ^ 4 A “ * 


j^'V "‘’ ' 
















Micro-H 


5.01 


Type of System , . 

Range. \ 

Useful Range 

Radar line- of- sight. For an aircraft altitude of 30,000 ft., the maximum 
range is about 230 nautical miles from the ground beacons. 

Accuracy and Precision 

The range of the aircraft from any beacon is accurate to about + 50 yards. 

Presentation of Data 

Visual presentation on PPI. 

Operating Skills Required 

Trained radar operator in the aircraft. No operators required at ground 
beacon sites. 

Equipment Required 

The aircraft must carry a weight of about 370 lbs. of AN/APS-15 radar 
equipment in addition to the 15- lb. Micro-H attachment. Two AN/ CPN- 6 ground 
beacons are required. 

RF Spectrum Allotments Required 

X-band radar (9335-9415 mcps). Bandwidth about 2.5 mcps. 

Present Status 

Operational. 

In the Micro-H system, a predetermined hyperbolic or circular course may 
be accurately maintained by an aircraft through the use of simultaneous range 
measurements to each of two ground responder- beacons. This method of triangula- 
tion from known beacon stations is capable of high precision since each beacon res- 
ponse is sharply defined in range, and no azimuth measurements are involved. Such 
a system, unlike Oboe, makes possible the use of beacon responses by many aircraft 
simultaneously; and in the case of Micro-H Mark II, these aircraft may fly on dif- 
ferent courses. The system is used primarily for blind bombing. 

Micro-H Mark I is a Micro-H system utilizing a time delay in the response 
of one of two ground beacons. Hyperbolic courses only may be flown in this system. 
No equipment other than AN/APS-15 (H2X) is required in the aircraft. The adjust- 
able delay in one of the ground beacon responses may be set to any desired value 
corresponding to a given hyperbolic course. The controlled aircraft flies along the 
desired hjrperbolic path by keeping the two beacon responses at the same apparent 
range on the PPI. This is easily done by following the beacon responses with the 
adjustable slant range marker. The bombs are released when the required range 
from the undelayed beacon is reached. With the Micro-H Mark I time delay equip- 
ment at one of the two ground beacon stations, all aircraft, using this pair of beacons 
for H- type navigation, are restricted to flying along a single hyperbola, the position 
of which is determined by the amount of time delay introduced at the ground beacon. 
The beacon containing the delayed response is useless for general navigation. 

Micro-H Mark II (AN/APA-40) is an attachment for AN/APS-15 (H2X) equip- 
ment which provides for the introduction of appropriate time delays in beacon res- 
ponse by circuits in the airborne equipment rather than at a ground beacon station. 
With this arrangement, an aircraft can fly either a hyperbolic or a cat-mouse course; 
and the choice of a particular hjqDerbola or a particular cat- mouse course is de- 
pendent only upon the settings of controls in the aircraft. The same ground beacons 


5.02 


Micro-H 


may be used by aircraft flying on different missions. 

For Micro-H Mark II, no additional time-delay circuits are required, other 
than the phantastrons already incorporated in the H2X equipment, but they need to 
be reshuffled somewhat for Micro-H operation. The AN/APS-15 (H2X) equipment 
is described inSection 22. The Micro-H Mark II attachment is essentially a switch- 
ing device which provides for different 
sweep- delays and range- mark delays in 
each of two 180® azimuth sectors of the 
PPI scan. The switching device is syn- 
chronized with the rotation of the antenna 
spinner so that the range measuring cir- 
cuits can be suitably adjusted for observa- 
tion of beacon A during half a revolution 
and for beacon B during the other half re- 
volution of the antenna. The mechanical 
motion of the antenna is transmitted by a 
synchro link to the Micro-H Mark II con- 
trol unit where it runs a system of cams 
and microswitches which periodically 
change the sweep and range delays as the 
antenna turns through the two 1 80® azimuth 
sectors at some point within each of which 
the antenna looks towards one of the bea- 
cons. The switching action may be made 
to occur at any desired azimuth, such as 
midway between the two beacons. When 
flying a hyperbolic course, the beacon res- 
ponses appear at the same apparent range 
as illustrated in Figure 5-01. 

The phantastron delay circuits are arranged as shown in the simplified 
block diagram of Figure 5-02. As in H2X, the step-delay phantastron can be made 
to open an eight- mile gate around any 10- mile crystal- controlled pip between 10 
and 200 miles in order to obtain a trigger pulse delayed by an integral number of 
ten- mile steps. In Micro-H Mark II however, the control- voltage for this step-delay 
phantastron is alternately taken first from one and then from the other of two 
voltage- dividers which control the ten- mile-step sweep delays for the two sectors. 
The triggering of the PPI sweep is still further delayed by the altitude phantastron 
which introduces a continuously variable delay of any desired fraction of ten miles. 
The range mark may be delayed from the triggering of the PPI sweep by the range 
phantastron which introduces any desired delay between about 0.6 and 16 miles. 
The calibrated continuously- variable delays introduced by the altitude and range 


/ 


/ 



Fig. 5-01 Beacon responses for a 
hyperbolic course 


MODULATOR SWEEP 



Fig. 5-02 Arrangement of delay circuits in Micro-H Mark H 



Micro-H 


5.03 


phantastrons are adjusted by the controls on the drum computer and also by the 
settings of two additional control- voltage potentiometers (one for each phantastron) 
located in the Micro-H Mark II unit. The latter two controls may be set for a de- 
sired delay and locked in position. Thus provision is made so that any of the phan- 
tastrons may introduce a different delay in the different sectors of the PPI scan, 
although depending upon the type of course flown, it is desirable for one of the phan- 
tastrons to introduce the same delay for both sectors. The arrangement and switch- 
ing of the delays depends upon the type of course to be flown. 



Cat-mouse course Cat-mouse course Hyperbolic course 

(cat beacon in B- sector) (cat beacon in A- sector) 


Fig. 5-03 Courses available with Micro-H Mark II 


Three types of courses as illustrated in Figure 5-03 may be flown with the 
aid of a Micro-H Mark II attachment for H2X. For cat- mouse courses the aircraft 
flies along a circular arc at a constant range from one beacon (the cat beacon) and 
releases bombs when reaching the proper range from the other (the mouse) beacon. 
The cat- mouse roles of the beacons may be interchanged to provide an alternative 
direction of approach. For this tjrpe of course, the altitude phantastron introduces 
the same delay in both sectors. When flying a cat- mouse course, the start of the 
PPI sweep in the cat sector is delayed in time by an appropriate amount such that 
the first return from the cat beacon appears at a convenient place on the expanded 
sweep- -say half way out. A fixed range mark is then set up so that it comes exact- 
ly at the desired cat range. In the mouse sector, the start of the PPI sweep is de- 
layed so that all of the bombing run of perhaps 15 miles appears on the expanded 
sweep. The range mark is adjustable within this mouse sector by means of the drum 
computer, so that the operator can check his mouse range occasionally as he 
approaches the release-point range. 

For a h 3 rperbolic course, the delay introduced by the range phantastron is 
the same for both sectors, while the delays introduced by the other two phantastrons 
are different for each sector. In the case of a hyperbolic course, the start of the 
PPI sweep in the sector containing the nearer beacon is delayed so that all of the 
bombing run occurs within the range of the expanded sweep. The start of the PPI 
sweep in the sector containing the farther beacon is delayed by this same amount 
plus the required difference in the ranges to the two beacons corresponding to the 
particular hyperbolic course flown. The drum computer controls the position of 
the range mark in both sectors, and the two beacons will appear to be at the same 
range if the aircraft is on course. 

With the Micro-H Mark II attachment for H2X, provision is also made for 


5.04 


Micro-H 


the use of sector scan in normal beacon navigation. When used in this way for bea- 
con navigation, the PPI sweep is delayed in each sector only in steps of ten miles to 
which must be added the appropriate range mark delay in order to obtain the range 
of a beacon in that sector. 


Bibliography 


Identification Classification Title 


M-197 Confidential Handbook of Maintenance 

Instructions for AN/aPA- 40 
(Micro-H Mark II) 


Section 4 Secret U. S. Radar Survey (pages 

Navigational 4-163 to 4-166) 

Radar 


Issued by 
MIT Rad. Lab. 


Div. 14 NDRC 


A.R.L. Intermittent Phase-Comparison Distance- 
Measuring System 


6.01 


Type of system , 

Pure range or "H" system. I 

Useful range 

36 miles for experimental model. 

Accuracy and precision 

Not known; probably better than i 1/2 mile. 

Presentation 

Dial indicates range (distance). 



CALIBRATED DIAL 


Fig. 6-01 Block diagram of system 


6.02 


Intermittent Phase-Comparison Distance- 
Measuring System 


Operating skill required 

(a) At ground beacon: can operate unattended. 

(b) In aircraft: none - read dial. 

(c) Time required to get reading: instantaneous - reading is correct every 
two seconds. 

Equipment required 

(a) At ground beacon: transmitter and receiver at frequency different by 
about 10 kcps. The transmitter may be a localizer transmitter adapted 
to simultaneously re- transmit the aircraft interrogation modulation-fre- 
quency of 2588 cps. Fairly simple to service. 

Radio-frequency spectrum allotments required 

Response - llOmcps - 6 kcps bandwidth. Interrogation - 100 or 120 mcps - 
6 kcps bandwidth. 

Present status 

Developmental. 

This distance meter is of the intermittent phase-comparison type. The phase- 
comparison method of distance-measurement was used by the Germans in their 
Benito system which is described in section 30. Figure 6-01 is a block diagram 
of the airborne equipment. An oscillator operating at 2588 cps supplies the voltage 
used for phase comparison. A timing circuit permits the transmitter to transmit 
for one twenty-fifth second every two seconds. This transmission is modulated by 
the 2588- cps audio frequency. The receiver is gated on during the time that the 
transmitter is transmitting. 

At the ground the interrogating signal is received and the recovered modu- 
lation frequency of 2588 cps is used to modulate a transmitter on a different fre- 
quency. The ground equipment is designed so that the phase shift is very small and 
constant. 

The Bell Telephone Laboratories published an unfavorable report on the use 
of the phase-comparison principle using existing communication equipment in 
ground vehicles. They stated that the condition of small and constant phase shift 
could not be satisfactorily met. It should be borne in mind that this should not con- 
demn the system as such, since the communication equipment tested worked at fre- 
quencies of only a few megacycles and had quite a narrow bandwidth. The resultant 
steep phase vs. frequency characteristic made this particular application of the 
principle impractical. 

The retransmitted signal from the ground is received at the aircraft and 
the phase of the recovered 2588-cps frequency is compared with the phase of the 
2588-cps voltage used to modulate the interrogating transmitter. The 2588 cps was 
chosen since 36 miles distance (total path-length 72 miles) will give a phase lag of 
360°. 


The phase comparison is accomplished by an automatic phase follow-up sys- 
tem. This system drives a calibrated continuous phase shifter in such a way that 
the output of the phase shifter will be kept in phase with the returning signal. 

Figure 6-02 is a diagram of the phase- sensitive thyratron motor-control 


Intermittent Phase-*Comparison Distance- 6.03 



Fig, 6-02 Phase- sensitive motor-control circuit 


circuit. The output of the phase shifter is shaped into a square wave and applied to 
the shield grids of the two thyratrons in opposite phase. The signal received from 
the ground transmitter is shaped into a square wave and applied to the grid of V3. 
On the negative swings of this voltage V3 is cutoff and the damped parallel circuit 
in the cathode starts a highly damped train of oscillations. The first swing will be 
negative. The cathodes of the two thyratrons are directly connected to the cathode 
of V3 so that the negative pulse is applied to these two cathodes. The plates of the 
two thyratrons are connected to opposite ends of the secondary of the 400-cps power 
transformer. Thecenter tap supplies one winding of the two-phase motor. The cur- 
rent in the other winding of this motor is shifted 90° by the use of a series capaci- 
tor. The phase of the current through the controlled winding of the motor and hence 
the direction of rotation of the motor depends upon which thyratron is firing. Fig- 
ure 6-03 illustrates the voltages applied to the shield grids and cathodes of the 
thyratrons. Either thyratron can only be fired by the negative cathode pulse if the 
positive half of the square wave is present on the shield grid. The square waves 
applied to the shield grids are not of sufficient amplitude to cut off the thyratrons 
once they have been fired. They are only extinguished when the 400 cps plate sup- 
ply voltage goes negative. As drawn in Figure 6-03 both thyratrons would fire and 




6.04 Intermittent Phase-Comparison Distance- 

Measuring System 



Square wave applied to 
shield grid of VI — ^ 


Square wave applied to 
shield grid of V2 ^ — 


Pulses applied to 
cathodes of VI and V2 


Fig. 6-03 Waveforms of phase- sensitive motor-control 


therefore there would be no resultant motor torque. If the pulses shift slightly to 
the right in phase only will be fired. The motor rotation produced will drive 

the phase shifter in such a direction that the two square waves are shifted to the 
right in phase and the balanced condition of Figure 6-03 is re-established. The 
calibrated dial indicates the phase shift required to bring about this balanced con- 
dition. 


The chief reason for developing such a phase-comparison distance-meter is 
the fact that it could tie in very well with an existing blind approach system. The 
localizer of this system radiates two lobes modulated by 90 cps and 1 50 cps res- 
pectively. It is thought that this transmitter and antenna system could be used as 
the ground responding transmitter in this system. The 2588-cps modulation could 
be radiated by both antennas. The localizer receiver in the aircraft could be used 
and the 2588 cps separated from the 90 cps and 150 cps by filtering. 


A.R.L. One-Shot Distance-Measuring System 


7.01 


Type of system 

Pure range or "H" system. ( 

Useful range 

50 to 100 miles. 

Accuracy and precision 

No values available. 

Presentation 

Veeder counter. 

Operating skill required 

(a) At the ground beacon: can operate unattended. 

(b) In the navigated craft: direct-reading Veeder counter. 

(c) Time to obtain a reading: instantaneous. 

Equipment required 

(a) At the ground beacon: Responder beacon - skill to service fairly com- 
plicated equipment. 

(b) In the navigated craft: Interrogator- responser and fairly complicated 
indicating system. 

Radio-frequency spectrum allotments required 
Not known. 

Present status 

Developmental. 

Description of system 

This system is of the type that transmits an interrogating pulse to a ground bea- 
con. The beacon receives this interrogating pulse and transmits a response pulse pre- 
sumably on a different frequency . The equipment in the aircraft measures the time de- 
lay between the interrogating pulse and the response pulse. The system to be describ- 
ed is \musual in that it uses the very low recurrence rate of 1 pulse per second. 
The circuit is designed so that it can measure distance using a single pulse; thus 
it can be called a one-shot system. The fundamental time-measuring circuit is a 
gated charging circuit for a capacitor as shown in Figure 7-01. Tube V 2 is fired 
by the transmitted pulse and Ci begins to charge. The received pulse fires Vj. 
This drops the plate voltage of V 2 to about -15 volts. Since Ci has acquired some 
charge the cathode of V 2 will be positive and V 2 will be cut off. The charge on Ci 
therefore depends upon the time between transmitted pulse and received pulse. The 
switchS 2 is openedto cut off Vj and make the circuit ready for another cycle. The 
switchSi is also closed to the bottom side to short Cj once each cycle. The voltage 
at point X indicates the distance. An electronic follow-up drives a calibrated volt- 
age divider so that its voltage equals the voltage at point X. Figure 7-02 is a block 
diagram of the follow-up circuit. The voltage from point X and the voltage from 
the voltage divider are applied to a balanced vacuum-tube voltmeter circuit (simi- 
lar to the voltohmist circuit) . The output of the voltmeter circuit controls a ring 
modulator circuit. A 400-cps voltage is supplied to the ring modulator circuit. The 
400-cps output will be of 0^ phase if the voltage of X is greater than that from the 
voltage divider. If the voltage of X is less than that from the voltage divider the 
output phase will be 180®. The amplified output of this ring modulator controls a 


7.02 


One-Shot Distance-Measuring System 



Fig. 7-01 Gated charging circuit 


thyratron phase- sensitive motor- control circuit. This phase- sensitive motor-cdn- 
trol supplies one stator winding of a two-phase motor. The other stator winding is 
supplied through a 90® phase shifter from the 400 cps supply. 

Figure 7-03 is a complete circuit diagram of time-measuring circuit and 
follow-up system. 

Figure 7-04 is a diagram of the phase relations present in the motor- con- 
trol circuit. Figure 7-04 (a) represents the phase relations present when the con- 
trol voltage is zero (zero error- voltage). The plates of V 7 and Vg are supplied 
with 400- cps voltages 180® out of phase. The cathodes are also supplied with 400- 
cps bias voltage 180® out of phase. The bias voltage of each tube is made to lag 
the plate voltage by approximately 135®. The control voltage is applied to the two 
grids in phase. When no control voltage is present both tubes will conduct for a 
short period each cycle. Since the two tubes will supply equal currents through 
the stator winding no torque will be produced. When a control signal is present 
as shown in Figure 7-04 (b) one tube will conduct for a longer period than the other ' 
due to the fact that one grid signal is shifted so that it lags the plate voltage by a 
lesser amount and the grid signal of the other tube is shifted so that it is more 
nearly 180® out of phase with its plate voltage. This will unbalance the current sup- 
plied the stator winding and a torque in one direction will be produced. When the 
phase of the control signal is reversed as shown in Figure 7-04 (c) the unbalance of 
conducting times is reversed and a torque in the opposite direction will be produc- 
ed. The two-phase motor will be driven in such a direction as to make the error 
voltage applied to the voltmeter circuit zero. This motor also drives a P.M. field 


One-Shot Distance-Measuring System 


7.03 


VOLTAGE REGULATED 
POWER SUPPLY 



Fig. 7-02 D. C. follow up 


400 CpS 
POWER 


7.04 One-Shot Distance-Measuring System 



Fig. 7-03 Circuit diagram of one-shot distance meter 



One-Shot Distance-Measuring System 


7.05 




Criticol Firing Voltage 



Criticol Firing Voltage 


Critical Firing Voltage 



Critical Firing Voltage 


Critical Firing Voltage 


Fig 7-04 Phase relations in motor- control circuit 


7.06 


One-Shot Distance-Measuring System 


DC generator. The output of this generator is applied in series with the voltage 
from the voltage divider which is applied to the voltmeter circuit. Since the polar- 
ity of this voltage is opposite to the error voltage and proportional to the speed of 
the motor it serves as an anti- hunt element. This is a typical "error plus first de- 
rivative" servo- system which is widely used in a variety of forms. The motor 
drives a Veeder counter which indicates the distance. 

Since quite short pulses may be used and because of the very low repetition 
rate, high peak powers can be used. It also means that a large number of aircraft 
can interrogate one beacon. 


Canadian Distance-Measuring System 


8.01 


Type of system 

Pure range or "H" system. ^ 1 

Useful range / 

50 miles at 1000 feet; 100 miles maximum range. 

Accuracy and precision 

(a) Calculated and estimated errors + 1 mile. 

(b) Errors revealed by operational use + 1 mile. 

Presentation 

Visual presentation on meter. 

Operating skill required 

(a) At ground beacon - may operate unattended. Skill to service responder 
beacon required, (b) In the navigated craft - Little skill required. Manual range 
search must be turned until lock- on light lights and then switch is thrown to auto- 
matic follow-up. (c) Time required to obtain a distance measurement - Time to 
read meter if follow-up is tracking; time to search and lock on: 30 seconds tol 
minute (limit is beacon coding time). 

Equipment required 

(a) At ground beacon: Fairly complex responder beacon. Weight 1000 to 
2000 lbs. (b) In navigated craft: Fairly complex 
automatic range follow-up. Weight 22 lbs. installed. 

Radio- frequency spectrum allotments required 
Frequency - 

Interrogation - 202 mcps airways 
222 mcps approach 
Response - 212 mcps 
Bandwidth - 3 to 4 mcps 

Present status 

Experimental. 

Description of system 

This system uses an interrogator- responser on the craft and a responder 
beacon on the ground. The pulse repetition frequency of the interrogator is 
approximately 200 pps. The interrogating pulses are 2 microseconds long and the 
interrogation frequency is 202 mcps for airways beacons and 222 mcps for runway 
approach beacons. The beacons respond with a 5-microsecond pulse on 212 mcps. 
An automatic range-tracking circuit tracks the beacon in range and gives a meter 
indication. A manual range search must be used originally to select the desired 
beacon. A rate- of- approach meter has also been developed and flight tested. 

Figure 8-01 is a block diagram of the interrogator- responsor. The pulse 
repetition frequency is determined by a free- running multivibrator. The output of 
this multivibrator is applied to a special sawtooth generator. The slope and 
linearity of the sawtooth are very constant. The slope of this sawtooth is negative. 
This sawtooth is applied to two circuits called "snaps". These snaps have the 
property of generating a pulse at the instant the sawtooth voltage equals an exter- 


interrogator- responser and 


subject to change 


8.02 


Canadian Distance-Measuring System 


LIJ 

^ f 


I- RECEIVER 


MULTI- 

VIBRATOR 


SAWTOOTH 

GENERATOR 


HIGH IMPEDANCE 
D.C. VOLTMETER 
DISTANCE indicator; 


'O 


DIFFER- 


ENTIA 


SNAP ^2 


GATED 

VIDEO 




— SNAP ^1 


GATED 

VIDEO 


5usec. 
PULSE GEN. 


lOjusec. 
PULSE GEN. 


MODULATOR 


TRANS- 

MITTER 


LU 


CONTROL 

AMPLIFIER 


INTEGRATOR 


INTEGRATOR 


- 









1 LOCK ON 

S.P.S.T. \ 


INDICATOR 


TORS 


T[^ 


S.Kb.l. \ 




LOCK ON 
LIGHT 


MANUAL RANGE 
SEARCH 


INVERTER 


AMPLIFIER 


SYNC. RECT. 



D.C. VOLTMETER 

RATE OF APPROACH INDICATOR 


Fig. 8-01 Block diagram of system 


imlly applied DC control voltage. A fixed bias is applied to snap 1 sufficient to make 
it fxre on the linear part of the sawtooth. This output pulse drives the modulator 
which ^ turn drives the transmitter. The output of snap 2 drives two pulse-genera- 
tors. The pulses produced by these two pulse- generators gate two video amplifiers 
connected to the receiver output. The two gates and received pulse are shown in 
Figure 8-02. The output of these two gated video amplifiers is integrated and mix- 
ed. The 10-microsecond gate gives a positive output voltage. The 5-microsecond 



Canadian Distance-Measuring System 


8.03 


5 jui-sec. Gate 
10 ;i-sec. Gate 
Received Pulse 


1 

Received Pulse falling in 
5 ju-sec. Gate tends to 
move Gates to Left. 

Received Pulse falling in 
10 jui-sec. Gate tends to 
move Gates to Right. 


Fig. 8-02 Video gates and received pulse 


gate gives a negative output voltage and has 20 to 30 times the effect of the 10**micro- 
second gate. The mixed output of these two gated channels is applied to a control 
amplifier. This control amplifier employs an electronically-amplified time- constant 
and will hang on for several minutes if the control voltage disappears. The output 
of this control amplifier controls snap 2. A positive voltage applied to the control 
amplifier results in an increase in the spacing of the pulses produced by the two 
snaps. The output of the 10-microsecond gated channel therefore tends to move the 
pulse of snap 2 farther from that of snap 1. The output of the 5-microsecond gated 
channel (with about 20 to 30 times the effect of the other) tends to move the gates 
closer to the pulse of snap 1. This results in an automatic follow up which will 
track the received pulse in range. Since the 5-microsecond gated channel has the 
greater gain the follow-up will lock on to the leading edge of the received pulse. 

When it is desired that the system track a particular response the SPST 
switch must be thrown to manual track and the manual range-search potentiometer 
varied until the beacon response falls into the 10 microsecond gate. The lock- on- 
light will light when this condition is reached. The beacon code may be read from 
the lock- on light. The switch may then be thrown to automatic track. 

Since the spacing of the gates from the pulse of snap 1 depends upon the 
difference in two DC voltages a voltmeter can be connected between these two 
points and used to measure the distance. 

The circuit of the linear sawtooth generator used to trigger the two snaps 
is given in Figure 8-03. This circuit, sometimes called a Miller Rundown, makes 
use of negative feedback to obtain a very linear sawtooth. A positive rectangular 
pulse is applied to the screen grid of V3 from the cathode follower which is driven 
by the timing multivibrator. This positive pulse on the screen grid increases 
the plate current and the plate voltage of V3 begins to decrease. This decrease 
is applied to the grid by the coupling capacitor connected from plate to grid. Thus 
the grid voltage is lowered and the plate current is prevented from rising to as 
high a value as it would in the absence of the coupling capacitor. Thus the plate 
voltage decreases slowly and very linearly. The output is taken directly from the 
plate of V3. 

The circuit given in Figure 8-04 is that called the "snap”. The sawtooth 
waveform from the Miller Rundown is applied through a decoupling filter and the 


8.04 


Canadian Distance-Measuring System 






To Snaps 



Fig. 8-03 Sawtooth generator 


secondary of the transformer to the cathode of V4. The plate of V4 is connected 
to +200 volts through a large resistor. This plate is also coupled to the grid of 
V5, As long as the sawtooth voltage is greater than 200 volts V4 cannot conduct. 
The plate current of V5 will be high since the only bias will be the contact poten- 
tial bias developed across the 2.2 megohm grid resistor. When the sawtooth 
voltage reaches 200 volts the diode V4 will begin to conduct and the grid voltage 
of V5 will start dropping. The plate current of V5 will start to decrease. The 
transformer B.O.-l is phased in such a way that the decreasing cathode current 
induces a voltage in the secondary of the polarity given on the diagram. This 
lowers the grid voltage still farther. This is a regenerative action resulting in a 
sudden plate- current cutoff. The plate-currentof the tube is then held at zero since 
the sawtooth is differentiated by the grid coupling circuit and holds the grid below 
cutoff. The positive pulse output is taken from the plate of V5. Two of these snap 
circuits are used. The one shown is the one that times the transmitted pulse. The 
other one controls the timing of the tracking gates. This second snap is the same 
as the one shown in Figure 8-04 with the exception that a variable control voltage 
is applied instead of the fixed 200-volt control voltage. 

Figure 8-05 is a diagram of the diode control- voltage comparator. The out- 
put of each gate is a negative pulse if the response pulse falls in the gate. The nega- 
tive pulse from the 5- microsecond gate is applied to the cathode of VI 6b through the 
.006->xf coupling capacitor. The negative pulse from the 10-microsecond gate is 
applied to the cathode of VI 6a through the .0005-^f capacitor. When the output of 
the 10-microsecond gate swings negative V16a conducts and the .0005-/if capacitor 
is charged with the polarity shown. When the negative pulse is over the cathode 
of V16a swings positive due to the charge on the .OOOS-yuf capacitor. This positive 
voltage is connected to the output through the 4.7-megohm resistor. When response 
pulses fall in the 5- microsecond gate VI 6b will conduct for each negative pulse 
and supply a negative control- voltage through the 4 70- thousand- ohm resistor. 


Canadian Distance-Measuring System 


8.05 



Fig. 8-04 Snap circuit 


4.7 Meg 



Control 

Voltage 


Fig. 8-05 Diode Comparator 


8.06 


Canadian Distance-Measuring System 


+ 285v. ■*-200v. 


+I20V. 

Regulated 




to MonuQl Range 
Search 



Control 

voltage 


Fig. 8-06 DC control amplifier 


The pulse from the 5-microsecond gate is therefore more effective in making the 
control voltage negative than the pulse from the 10-microsecond gate is in making 
the control voltage positive. In order to attain a stable intermediate control 
voltage the output pulse from the 5-microsecond gate must be very short. 

Figure 8-06 is a diagram of the control amplifier that controls the variable 
snap. The 4/if-coupling capacitor between plate and grid of V7 gives this tube an 
amplified time-constant effect. The functions of this control amplifier are to retain 
temporarily the condition established by the most recent control pulses, to smooth 
out pulsations from the control voltage and to bridge gaps due to short-period 
interruptions. The control voltage is applied to the grid. If this control voltage 
is removed the grid voltage will change very slowly since the only DC return of 
this grid circuit is the leakage resistance. If for instance the grid voltage tended 
to increase the plate voltage would decrease and this decrease could be coupled 
back to the grid through the 4 ;>if. coupling capacitor. This grid is tied directly 


Canadian Distance-Measuring System 


8.07 


to the grid of V6 which acts as a DC amplifier to control the triggering voltage on 
the variaWe snap. The plate voltage of V6 is the bias voltage of the variable snap. 
The time delay between the triggering of the fixed snap and the variable snap is a 
function of this bias voltage. As this bias becomes less positive the time delay in- 
creases. If the response pulse falls only in the 10-microsecond gate the control 
voltage developed is positive. The bias applied to the variable snap is therefore 
made less positive and the time delay between the transmitted pulse and the track- 
ing gates increases. This delay will increase until the leading edge of the response 
pulse enters the trailing edge of the 5 -microsecond gate enough to being about a 
balanced condition. 

The complete circuit diagram is given in Figure 8-07. 

The part of Figure 8-01 enclosed in the dotted line is the circuit for produc- 
ing the rate- of- approach indication. The DC bias voltage and the slowly- changing 
bias voltage applied to the distance meter are applied to two differentiators having 
a time constant of 1 second. A differentiator is used on the DC bias voltage to 
eliminate errors due to voltage fluctuations. The outputs of these two differentiators 
are applied to a vibrating- reed inverter and converted to AC. This AC is amplified 
in a stable amplifier. The output of this amplifier is rectified by a vibrating- reed 
synchronous rectifier and applied to a DC meter which indicates ^ate- of- approach. 
The heading for zero rate- of- approach can be determined to + 1 or better. The 
heading for maximizing the rate- of- approach can be determined to + 6® or better. 


8.08 


Canadian Distance-Measuring System 



Fig. 8-07 Circuit diagram 


Identification 


Canadian Distance-Measuring System 


Classification Title 

Confidential Airborne Distance 
Indicator 


8.09 


Issued by 

National Research 
Council of Canada 
Radio Branch 
15 July 1945 





GE Random Interrogation Distance Measuring System 


9,01 


Type of system 

Pure range or "H" system. 

Useful range 

Not known - probably 100 miles. 

Accuracy and precision 

Not known. 



Fig. 9-01 Block diagram 







9.02 


GE Random Interrogation Distance Measuring System 


Presentation 

Veeder counter. 

Operating skill required 

(a) At ground beacon: can operate unattended. 

(b) In the navigated craft: no special skill required- -automatic presenta- 
tion. 

(c) Time required to obtain a reading: instantaneous. 

Equipment required 

(a) At ground beacon: responder beacon - requires highly trained personnel 
to service. 

(b) In the navigated craft: interrogator, receiver and automatic follow- up. 
Fairly complicated and requires highly trained personnel to service. 

Radio frequency spectrum allotments required 

Not known. 

Present status of development 

Proposed. 

Description of system 

In this system the equipment on the navigated craft interrogates a ground 
responder beacon and measures distance by measuring the delay between the trans- 
mitted pulse and the beacon response. An unstable oscillator supplies the syn- 
chronization for the system. It triggers the modulator which contains a pulse shap- 
ing circuit. The modulator in turn pulses the transmitter. An automatic range 
follow-up system is used. The output of the receiver is applied to an early gated 
amplifier and to a late gated amplifier. The outputs of these gated amplifiers 
which are of opposite polarity are compared in the diode comparator and the differ- 
ence used to control a biased multivibrator delay circuit through a DC control 
amplifier. The fast sawtooth is triggered by the synchronizing oscillator and is 
applied to the biased delay multivibrator. The delay produced is directly propor- 
tional to the bias voltage from the DC control amplifier. When the tracking gates 
are locked on the response pulse the DC bias applied to the delay multivibrator pro- 
duces a time delay equal to the delay between transmitted pulse and response pulse. 
When the delay between the transmitted pulse and the response pulse changes, the 
bias on the delay multivibrator changes and the delay produced by the delay multi- 
vibrator changes so as to make the tracking gates follow the response pulse. An 
automatic range search is obtained by switching the DC control amplifier to a slow- 
range search sawtooth. When the gates are moved to a delay such as to accept 
response pulses the relay amplifier is energized and switches the DC control ampli- 
fier over to the automatic follow-up gates. Presumably a DC follow-up is used to 
drive the Veeder counter distance-indicator. 


GE Time- Rationing Distance- Measuring System 


10.01 


Type of system 

Pure range or "H" system. 

Useful range 

loo miles maximum. 

Accuracy and precision 
Not known. 

Presentation 

Meter. 


/ 


Operating skill required 

(a) At ground beacon: Operates unattended, (b) In the aircraft: No special 
skill required, (c) Time to obtain a fix: Instantaneous. 


Equipment required 

(a) At ground beacon: Responder beacon and 800-cps timer, (b) In the air- 
craft: Interrogator-responser and fairly complicated control circuit. 


Radio-frequency spectrum allotments required 

Not known. 


Present status 

Proposed. 



Fig. 10-01 Block diagram 



10.02 


GE Time- Rationing Distance-Measuring System 


Description of system 

This system uses an interrogator- responser on the aircraft and a responder 
on the ground. This system is unusual in the fact that the interrogations are 
invited by the ground beacon. The ground beacon transmits 2-microsecond invi- 
tation pulse sat a rate of SOOpps. In the aircraft equipment the 2-microsecond pulses 
are selected by a pulse -len^h discriminator. These pulses are applied to a fre- 
quency divider which steps the frequency down to 16 cps. The aircraft interrogates 
at a 16-pps rate. There are therefore 50 time-channels available for 50 interro- 
gating aircraft. In the aircraft equipment there is an automatic search circuit 
which finds an empty time channel and transmits its interrogating pulses in that 
channel. The beacon responds to an interrogation with a 1- microsecond pulse. 
These 1-microsecond pulses occurring at a 16-pps rate are separated from the 
2-microsecond invitation pulses by a pulse-length discriminator. Since the dis- 
tance corresponding to the period of an 800-cps frequency is 116 miles the maxi- 
mum distance is limited to 100 miles. 

A block diagram of the craft equipment is shown in Figure 10-01. 


GEE 


11.01 


GEE (OR G) SYSTEM OF NAVIGATION 


Type of system: Differential range (hyperbolic) / 

Useful range and coverage area 

Since the frequencies used with Gee are of the order of 20-85 mcps., reception 
is essentially limited to line- of- sight range, although refraction and ducting extend 
this somewhat. Gee fixes have been obtained by sky-wave, but this is not to be relied 
upon. The range practically attainable therefore depends mainly upon the height of 
the craft and the siting of the transmitter. This statement assumes that adequate 
power is radiated for good signal- to- noise ratio at ranges corresponding to the maxi- 
mum operational craft heights encountered. Another way of stating this is to remark 
that if a given average radiated power provides satisfactory reception at a line-of- 
sight range corresponding to a craft height of (say) 20,000 feet, then the range at this 
and lower altitudes will not be materially increased by increasing the transmitted 
power. 

Formulae are available for computing ranges at various heights (see section 
2 of this document). 

Line- of- sight ranges for various craft altitudes, assuming favorable trans- 
mitter locations (on the forward side of a high hill) but making no allowance for duct- 
ing and refraction effects, are as follows: 

Height of craft (feet) Range (statute miles) 


126 

161 

212 

250 

283 


5000 

10000 

20000 

30000 

40000 


As previously mentioned, the effect of refraction and ducting will be to increase 
these figures, so that at 10,000 feet (for example) the range might be extended from 
161 miles to 250 or 300 miles. As a result of this, the maximum operational range 
of Gee at heights of 30^000 feet and over has been considered as about 400 miles. 

Accuracy and Precision 

It is shownlater in this discussion that time measurements with the Gee indi- 
cator can be made to within 2/3 microsecond under good conditions. Position- line 
precision is greatest along the base-line (see Fig. 11-02). In this location, an error 
of 2/3 microsecond in time measurement produces an error of 0.062 mile = 327 feet 
in line of position. As with all hyperbolic systems, the error in line of position re- 
sulting from a given error in time measurement varies according to the craft' s 
position on the family of hyperbolic position- lines. This subject is discussed else- 
where (see page 12.10). Error in a fix is discussed in section 1. These errors are 
theoretical. Operational data indicate an average accuracy of 2 to 3 miles in position- 
line determination (average of many results under varying conditions). 

Type of presentation 

Visual. Pulse alignment and time- marker counting on a cathode- ray tube. 
Operating skill required 

(a) Ground installations: A full-time trained monitor operator is required 
for each Gee station, (b) Craft: The operator must have instruction in the use of 
the specialized equipment. The actual operations follow a routine and are in them- 


11.02 


GEE 


selves simple, (c) Time to obtain a fix: Approximately 1 minute. Runningfix 
technique is not required. 

Equipment required 

(a) GroundTnstallations: Each station consists of a pulse transmitter, with 
appropriate timing circuits and test gear. One chain of four fixed stations gives a 
fixover its coverage area. The fourth transmitter makes accuracy possible in areas 
where the accuracy of fix obtained from the other three would be poor. Craft: A 
specialized Gee receiver and indicator are required, together with Gee charts. 

Frequency and wavelength 

The system has been used on frequencies within the 20-85mcps band (15- 
3.5 meters). The bandwidth of the receiver is about 1 mcps. 

Present Status 

Gee has been the standard British aircraft electronic navigational aid used 
during the war. It was the principal navigational aid used during the initial landings 
in France on D-Day, 1945. German use of Gee transmissions is known as Hyperbol. 

Principle of operation 

The four ground stations comprising a chain transmit on the same frequency. 
The four stations are here designated A, B, C and D. A is the "master” station and 
the others are "slaves". The A station transmits pulsesof 2-10 microseconds 
width with a pulse repetition frequency of 500 pps. Stations B and C transmit pulses 
with a repetition frequency of 250 pps, the two stations being synchronized to alter- 
nate pulses from the A station. The exact synchronization of the slave stations to the 
transmission from the master station is essential to the accuracy of the system and 
represents the manual control mentioned above. The D station transmits double 
pulses, and has a repetition frequency of 500/ 3 pps. 

The transmissions from the slave stations are triggered by the master sta- 
tion. That is, pulses radiated from the master station (where the prf is accurately 
controlled by a carefully stabilized crystal oscillator) arrive at a slave station, are 
received and cause the slave station to emit pulses of its own. The exact timing of 
the pulses will therefore depend on the distances between the master and each of 
the slave stations. This distance is of the order of 70 - 80 miles, representing a time 
of transmission of about 400 microseconds. To this there is added at the slave station 
a delay- time which is a constant and which represents a controlling factor in the 
location of the position lines obtained. 

Assuming (for purposes of illustration) that a time interval of 500 microsec- 
onds elapses between the emission of a pulse from the master station and emission 
of the triggered pulses from any of the three slave stations, the sequence of events 
would be as shown in Figure 11-01. (Note that 500 microseconds = 0.5 milliseconds). 
Certain of the A pulses are double: this point is referred to later. 

A Gee receiver at some definite location will receive signals from all four 
transmitters, but the time relations existing among the received pulses at the location, 
of the craft will not be the same as in the diagram of Figure 11-01 on account of the 
different distances between the four transmitters and the receiver. It is by measure- 
ment of the time delays between the A, B, C and D pulses that a fix is obtained. To 
amplify this statement, consider the hypothetical location of the four transmitting 
stations represented in Figure 11-02. 

A craft located anywhere on the line a^bQ (the perpendicular bisector of the 
line AB joining the A and B transmitters) will receive the A and B pulses in the 


GEE 


11.03 


/ 


A 

PULSES 


time 0 
(millisec) 


B 

PULSES 


10 


12 


C 

PULSES 


D 

PULSES 


1 


Fig. 11-01 


same time relationship as that obtaining between the original transmissions, since 
both signals will have experienced the same time delay in reaching the craft. This 
line is therefore the locus of all points for which the relative time delay between the 
received A and B pulses is constant and equal to that between the transmitted pulses. 

Another line a^b^ may be drawn such that for any point on it the relative time 
delay between the received A and B pulses is some other fixed amount. This line is 
a portion of a hyperbola, of which A and B are the foci. There is an infinite family 
of such hyperbolae, each one characterized by a definite fixed time delay. Thus if the 
operator on the craft can determine the relative time delay between the received A 
and B signals, and also which signal came first, he may locate himself as being on 
one of the hyperbolic position lines. This function is performed by means of the Gee 
receiver and indicator, which displays the received pulses on a suitable time-base 
which can also be furnished with time- marker pips. The problem of determining 
which signal arrived first is avoided due to the fact that slave pulses are triggered 
by the arrival of master pulses. There is thus no point at which the slave pulse can 
arrive first. 

Observations on pulses from stations A and C will likewise give a set of hyper- 
bolic position lines (dotted lines in Figure 11-02). The intersections of these two 
families of hyperbolae yield a set of fixes. Gee charts are provided on which these 
hyperbolae are overprinted, different colors being used for different pairs of stations. 

There are certain intersections in this lattice of intersecting hyperbolae at 
which the angle of intersection is so acute as to lead to considerable possible error 
in the fix obtained. In these positions, observations from stations A and D are used 
in place of either the A-B or the A-C set of pulses. Station D is so positioned as to 
give the necessary coverage in these areas. Constructions for areas of coverage, 
variations in precision in different regions on a family of hyperbolae, and sources of 
error will be discussed in connection with the Loran system. Gee and Lor an are both 


11.04 


GEE 



Fig. 11-02 Intersecting Gee lattices 


GEE 


11.05 


hyperbolic navigation systems and depend on the same principles, differing mainly 
in the frequency used and coverage area attained, and in certain details of the indi- 
cator circuits. The general considerations relating to hyperbolic systems are dis- 
cussed more fully in the Loran section of this report, for the reason that this materi- 
al is more easily available in various Loran publications. 

Gee System Transmitter 

The transmitter must be capable of putting out short pulses of RF energy 
and must be very accurately synchronized. At master stations, the transmitter is 
synchronized to pulses obtained from a crystal- controlled frequency- divider rack. 

At slave stations, synchronization is from the output of a receiver which picks up 
the pulses from the master station. A suitable fixed time delay is introduced between 
the output of the receiver and the pulsing circuits. 

The signal received by the receiver at slave stations will be weak, and it is 
essential that the performance of the receiver shall not be affected by spurious sig- 
nals from the slave transmitter itself. This transmitter must therefore be of the 
type in which the main oscillator is pulsed, so that this oscillator will not be radiat- 
ing at the time of arrival of the next synchronizing pulse from the master station. 

Furthermore, the synchronization of the transmitted pulses must not be sub- 
ject to random variations due to changes in the small but finite time required for an 
oscillator to build up from zero when pulsed. For this reason, a priming oscillator 
isused, very lightly coupled to the main oscillator. This priming oscillator is itself 
pulsed slightly in advance of the pulse which permits the main oscillator to oscillate. 

Monitoring is provided by a fixed cathode ray oscillograph whose sweep is 
initiated by the priming pulse. The vertical deflection plates of this oscillograph 
may be connected to various test points throughout the transmitter. Provision is also 
made for triggering the sweep of an external cathode ray oscillograph. 

Figure 11-03 shows a block diagram of the transmitter and associated timing 
circuits (omitting power supplies and control equipment) . 

The pulse- shaping tube is a pentode amplifier which is normally cut off. 
The positive pulses applied to its grid are large enough to draw grid current. The 
output from the plate is therefore a large, square negative pulse. A part of this out- 
put is applied to Vfj, whence it is used to trip the priming oscillator, the sweep of the 
monitor CRO and also an external monitor sweep if desired. V2 is normally conduct- 
ing and has its grid leak returned to the Bf line. It is a pentode, and the screen and - 
plate resistors are of large value so that, the screen and plate potentials are normally 
of the order of 10 - 15 volts above ground. The plate and screen are direct- coupled 
to the control grids of V3 and V4 respectively, and in both plate and screen circuits 
there are shunt RC combinations of variable time- constant. When therefore the grid 
of V2 is cut off by the arrival of the negative pulse from V^, its plate voltage rises 
exponentially with a time constant which is adjustable (preset "delay" control). Since 
the cathode of V3 is held constant at approximately +50v, a certain time elapses be- 
fore V 3 starts to conduct. This delay (adjustable) is of the order of 3-8 microseconds 
and represents the delay between the pulsing of the priming oscillator and that of the 
main oscillator. This and other waveforms are shown in Figure 11-04. WhenV3 
conducts its plate voltage drops sharply. This provides a timing edge to which the 
pulsing of the main oscillator is synchronized. The magnitude of the change in plate 
voltage of V 3 is controlled by having its plate supply voltage variable. This provides 
a control over the width of the pulse actually used to trip the main oscillator ("width 
trim 1"). V4 is a similar stage to V3, but fed from the screen of V2 instead of from 



itS: 

UJ z ^ 
►- 0 »" 
X z 
UJ 


Fig. 11-03 Block diagram — transmitter 





GEE 


11.07 


} 

V| GRID 


V| PLATE 


V2 PLATE 
V3GRID 


PLATE 


V2 SCREEN 
V4GRID 


V4 PLATE 


V5 GRID 


Ve GRID 


Vg PLATE 


Vii PLATE 


V|0 GRID 



Fig. 11-04 Transmitter waveforms 


GEE 


IIM. 

the plate, and having a longer time constant in its grid circuit than is the case with 
V3. The negative timing edge produced at the plate of V4 will therefore be delayed 
with respect to the timing edge produced by V3. This delay is of the order of 10-20 
microseconds and is adjustable by changing the screen resistor of V2 ("spacing" 
control). The reasonfor this second timing edge is to enable the transmitter to radi- 
ate double pulses if used at a "D" slave station. V4 may be disabled (by removing its 
screen voltage) if double pulses are not required. The depth of the timing edge pro- 
duced is likewise adjustable by varying the plate supply voltage to V4 ("width trim 2"). 

The outputs of V3 and V4 are differentiated and combined at the grid of V5. 
The grid resistor of V5 is returned to the Bf line so that the tube is normally con- 
ducting. Positive pulses therefore occur at the plate of Vg, synchronized to the tim- 
ing edges produced by V3 and V4. Further provision for varying the width of these 
pulses is made by changing the time constant of the grid circuit of Vr. The output 
from V5 is applied to Vc and the tops of the pulses are squared by gricf-circuit clip- 
ping. The plate circuit of Vg contains an inductance and is fed from a supply at about 
750v. Its output therefore consists of negative pulses of about 700v, with an over- 
shoot (due to the inductance) on the trailing edge. Its output is applied to the modu- 
lator tube andtheovershoot results in a sharp cutting-off of the RE pulses emit- 
ted by the transmitter. The modulator has its cathode held at -2000v, and its 
plate resistor returned to ground. This plate resistor is of 3000 ohms, and serves 
also as the grid leak of the oscillator stage. runs at zero bias and therefore 
draws a large current (over 0.5 amp). Its plate voltage with respect to ground is nor- 
mally -1600, which is sufficient to cut the oscillator off. When the 700v. negative 
pulses are applied to its grid, corresponding positive pulses appear at the plate and 
in this way the oscillator is pulsed. 

The oscillator (V12, V13) is of the push-pull TGTP type. When it is pulsed, 
there is sufficient coupling with the priming oscillator (which is already oscillating) 
so that the build-up time is short and of constant duration. The power amplifier 
stage (Vj4, Vic) is also push-pull and is fed by a tuned section of transmission line 
from the oscillator plates. The antenna is coupled to the power amplifier either by 
a tapped coil or by a tapped length of short-circuited transmission line. 

The priming oscillator (V^q) is a Hartley circuit, pulsed by the output from 
Vi taken through a suitable inverter and cathode follower (V^, V3). The grid circuit 
of V7 is arranged to have a time constant such that the tube is cut off for about 50 
microseconds by the negative pulses from V^. This is the duration of the pulses 
applied to the priming oscillator, which will therefore continue to oscillate after the 
main oscillator has stopped. But when a double pulse is required, the priming oscil- 
lator will still be functioning for the second pulse. Since the pulse repetition period 
is 4000 microseconds (at slave stations) the priming oscillator will be cut off long 
before the receiver is sensitized for the reception of the next transmitted pulse from 
the A station, and no difficulty will be experienced with feedback. 

The plate supply to the final power amplifier stage is variable from 2000v to 
to 28,000v and the maximum peak power output is about 300 kw. 

The Gee Receiver and Indicator. Principles of Operation 

(1) Basic Timing Device; 

The craft equipment comprises a superheterodyne receiver incorporating certain 
anti- jamming features, together with timing circuits and a cathode- ray tube in- 
dicator. 

The equipment measures accurately the time interval between the arrival 
of an A (master) pulse and that of a B (slave) pulse, and simultaneously the time 


GEE 


11.09 


interval between A and C pulses. Since both these measurements arfe made after 
the same set of adjustments, and at the same time, ideal conditions exist for 
obtaining a fix at a definite instant in time. 

Thebasic timing device at the craft is a 75-kcps crystal oscillator whose 
frequency is adjustable within a narrow range. This oscillator has its output 
circuit tuned to 150 kcps. and performs two main functions: 

(a) It provides two sets of timing markers on the display, at 1 50 kcps and 1 5 kcps 
frequency respectively (6.67 and 66.7 microsecond intervals between mark- 
ers). Furthermore, when using either set of markers, every fifth marker is 
raised to facilitate counting. 

(b) It is used to synchronize the sweep frequency to 500 cps. 

Since the prf of the master station is 500 pps. and that of the slave stations 
250 pps (A and B) or 500/3 pps (D), it follows that the received pulses will re- 
main locked in stationary positions on the display if, and only if, the frequency 
of the crystal in the craft is adjusted (by means of the fine frequency control) 
to agree exactly with that of the crystal at the master station, where the prf is 
accurately controlled. It follows therefore that the accuracy of the timing mark- 
ers is automatically that of the timing gear at the master station. The latter is 
of course very carefully controlled. 

(2) Display : 

The display in the Gee indicator consists of a linear sweep on which the pulses 
and time markers are displayed. In order to lengthen the sweep and so permit 
more accurate time measurement, the sweep is divided into two parts, which 
appear as two horizontal lines, one vertically under the other. The frequency of 
theentiredisplay is therefore 250 cps. Referring to Figure 11-05, the actual mo- 
tion of the CRT beam is as follows: 

P to Q: beam on, first half of ^weep, one A and one B pulse displayed.^ 

Q to R: return trace, beam blanked. 

R to S: beam on, second half of sweep, one A and one C pulse displayed. 

S to P: return trace, beam blanked. 

(3) Clearing Switch: 

By means of a two-position selector known as the clearing switch, either, time- 
marker pips or received pulses are displayed on the sweep, but not both. 

(4) Sweep Speeds: 

Three sweep speeds are provided. These are: 

(a) Main sweep, 250 cps, in two horizontal sections. This is used for approxi- 
mate pulse alignment, and for counting the whole number of 15-kcps time 
markers between A and B or A and C pulses. 

(b) Strobe sweep. This is much faster, and is divided into four parts, one for 
each of the four pulses visible on the display. It is used for accurate pulse 
alignment, and for counting the number of 1 50-kcps time markers (or tenths 
of 15-kcps intervals) in the time intervals to be measured. The position of 
the time intervals corresponding to the strobe sweeps is indicated relative to 
the main sweep by a small depressed section of the main sweep when this is 
in use. By means of fine and coarse controls, these time intervals corres- 
ponding to the B and C strobe sweeps can be shifted relative to the main sweep. 

D pulses also appear during alternate sweeps on each trace, since the prf of the 
D station is 500/3 pps. For purposes of explanation only the B and C pulses are 
presently considered. 


11.10 


GEE 



Fig. 11-05 Sweep sequence 


Since time markers still appear on the strobes, the fine and coarse controls 
need not be linear in themselves, but need only be stable over a period of 
time. 

(c) Expanded strobe sweep. Faster than the strobe sweep, likewise divided into 
four parts, used for final pulse alignment and for estimation of tenths of the 
spacings between 150-kcps markers (or hundredths of 15-kcps intervals). 

The appearance of the indicator under various conditions, illustrating the de- 
tails of the three sweeps available, is shown in Figure 11-06. The ghost pulse beside 
the A pulse on the lower trace is used to identify the two parts of the display, so that 
the operator may know which of the slave pulses should be positioned on the lower 
trace. 

Accuracy of Time Measurement 

It will be seen that by the above means, time intervals may be estimated to about 
one-tenth of a 150-kcps time-marker interval, or to 2/3 microsecond, if the pulse 
alignment and estimation are done with care. The sharpness of the pulses to be 
aligned (and therefore the accuracy with which they may be so aligned) depends on 
the band width of the receiver and also on the transmitter characteristics. With pre- 
sent equipment, it seems probable that the errors introduced due to unsatisfactory 
pulse- shape will be smaller than the residual error in estimation of time intervals, 
having regard to the radio-frequency used. 

Error in the master oscillator used for timing the pulses transmitted from 
a master station should be extremely small if the crystal temperature is closely con- 
trolled, so that this source of error is negligible compared to the others mentioned. 

There remain errors due to propagation conditions. These may be serious 
under certain conditions. They are of the same nature as those encoimtered in other 
hyperbolic systems such as Loran, and are discussed in more detail in section 1. 

Procedure used in obtaining a fix 

Starting with the main sweep and with received pulses displayed, the fine fre- 


GEE 


11.11 



(a) Main time-base, clearing 
switch up, signals on strobes 



I 



(b) Strobe time-base. 


pulses correctly aligned 



(c) Expanded strobe time-base, 
clearing switch down 


(d) Strobe time-base, 
clearing switch down 



(e) Main time-base, 
clearing switch down 


Fig. 11-06 Indicator displays 


11.12 


GEE 


quency control of the crystal oscillator is first adjusted so that pulses remain sta- 
tionary, with the A pulses at the left end of the traces. The coarse B and C strobe 
controls are now adjusted until the strobe markers on the main sweep are so posi- 
tioned that the B and C pulses stand on them. Switching to strobe time base, so that 
those intervals which were formerly strobe markers on the main sweep now are them- 
selves expanded into full sweeps, the fine B and C strobe controls are now adjusted 
so that the B and C pulses are aligned with the two A pulses, all four pulses lying 
vertically one under the other. This process is completed using the expanded strobe 
sweep. 


The clearing switch is now moved so that time markers are displayed instead 
of received pulses. 150-kcps markers are used first, the tenths and whole numbers 
of markers being counted with the expanded strobe and strobe sweeps respectively. 
The time of the fix is noted at the instant when the tenths are read. Finally, returning 
to the main sweep the number of whole 154ccps time markers is counted. The final 
count therefore gives the time intervals between the A and B pulse positions, and 
between alternate A and C pulse positions, to 1/100 of a 15-kcps time-marker inter- 
val. 


It will be seen that with this system, the time interval read is not directly 
that between pulses, but rather that between strobes which have been positioned so 
that the pulses lie in corresponding positions on them. The two operations requiring 
care (matching of pulses, counting of time intervals) have therefore been separated 
so that the full concentration of the operator may be exerted on each. It will be noted 
that the receiver gain used is the same for all received pulses. This means that for 
certain positions of the craft, in which the craft- station distances (and therefore the 
amplitudes of the corresponding received pulses) are of considerably different value, 
theheightsof the displayed pulses will not be the same. Since leading edges of pulses 
are to be matched, and since the slope of a leading edge is influenced by the peak 
height of the pulse, this feature imposes a certain limitation on the accuracy attainable. 

On the other hand, both sets of pulses are displayed simultaneously, so that 
a fix is obtained at a definite instant, and the time required to make the adjustments 
for a fix is less than in the case of systems where the two position- lines necessary 
for a fix are separately obtained. 

Indicator Circuits 

Figure 11-07 shows a block diagram of the timing and indicating circuits. 

The master oscillator (Vj 2 ) crystal controlled with a 75 kcps crystal in 
the grid circuit. A variable condenser in parallel with the crystal gives fine 

control of frequency. The plate circuit is tuned to 150 kcps. Vi 3 is a blocking os- 
cillator (squegger) whose output is a series of sharp pulses at 150 kcps. is a 
second blocking oscillator arranged to give a frequency division of 5. is a 

similar stage dividing by 2. likewise divides by 5. The 3 kcps output from 

Vj 0 drives a multivibrator (V^r^V^g) which may be switched to divide by 5, 
6 , or 7. Division by 6 is the normal arrangement, yielding a 500-cps pulse output. 
Vg and Vg form a square- wave generator, giving a 250-cps square-wave output. Yrj 

and V^Q are identical stages. The output of each is a rectangular (positive) pulse 
whose leading edge is locked to the trailing edge of a half- cycle of the 250-cps. square- 
wave input. V7 and VjQ are driven by opposite phases of the square-wave. The dur- 
ation of the output pulses is accurately controllable, coarse and fine adjustment being 
provided on the control panel in each case. 

These pulses (known as strobe timing pulses) together with the output of the 
500 cps multivibrator V 17 Vis may be used to initiate the cathode-ray sweep. Vn 


GEE 


11.13 



Fig. 11-07 Block diagram- -indicating and timing circuits 

is a stage in which all the pulses to be used for initiating the sweep are combined 
together. The 500-cps multivibrator pulses are applied through a clamping circuit 
to the suppressor grid of a pentode, while the control grid receives both the B and C 
strobe timing pulses. As a result the output at the plate of is a combination of 
three sets of positive pulses: (1) the A strobe pulses, 500 cps, leading edge locked 
to the trailing edge of the 500-cps multivibrator output, (2) the B strobe pulses, 250 
cps, leading edge delayed (as explained in connection with Vrj and V^q) behind the 
trailing edge of the 500-cps multivibrator output, (3) the C strobe pulses, 250 cps, 
similarly delayed but by a different interval and following alternate 500-cps multi- 
vibrator pulses. This output is sketched in Figure 11-08. 



fil 

m 


[FI 

[FI 


[¥1 

m 


"cT 

tl 



t2 


->■ 




+2 



Fig. 11-08 Sweep initiating pulses 
tj controlled by B strobe timing controls (coarse and fine) 
tg controlled by C strobe timing controls (coarse and fine) 


The waveform at the screen of Vn will be similar but without the A pulses, 
since changes of suppressor grid voltage do not materially affect conditions in the 
screen circuit. 

The time- base generator may be triggered either by the pulse output just des- 
cribed (strobe position) or directly by the 500-cps multivibrator pulses (main position) 
This generator (V^ Vg Vg) is of the type which produces a sawtooth sweep correspond- 




11.14 


GEE 


ing to each pulse supplied to it. The initiating pulses (or gating pulses) are applied 
to the suppressor grid of a pentode having a condenser connected between plate and 
grid. The grid- leak is returned to a point at some positive potential. When the sup- 
pressor grid is gated by a negative pulse, the condenser charges up. When the sup- 
pressor is returned to zero potential, the condenser discharges at a rate determined 
by the positive potential of the point to which the grid leak is returned. A sawtooth 
sweep is therefore produced for each pulse applied, the start of the sawtooth coin- 
ciding with the trailing edge of the initiating pulse. The output is push-pull so that 
both sets of horizontal deflecting plates are driven. The following sweeps are avail- 
able: 

(1) Sweep selector in "main” position, grid leak returned to a point of low positive 
potential, 500 sweeps per second. Each sweep is of about 2000 ^sec. duration. 

(2) Sweep selector in "strobe" position, grid leak returned to a point of high positive 
potential, 1000 sweeps per second of which 500 will be A strobe, 250 B strobe 
and 250 C strobe. Each sweep is of about 80 ymsec. duration. 

(3) As in (2) but the size of the condenser is reduced, giving an "expanded strobe" 
sweep. Each sweep is of about 20 yusec. duration. 

These displays may be identified in Figure 11-06. 

The 250-cps square wave from Vg Vg is applied to one of the vertical deflection 
plates. The main presentation therefore shows two horizontal time bases, one above 
the other. In addition, those sections of the main time base corresponding (in time) 
to the Band C strobe sweeps are slightly displaced downward. This is accomplished 
by V 3 , whose action includes several functions now to be described. 

Two types of deflecting voltage can be applied to the upper vertical deflection 

plate: 

(1) The output of the receiver (positive pulses) containing received pulses from the 
A station (500 per second) from the B and C stations (250 per second) and from 
the D station (double pulses, 500/3 per second). 

(2) Calibration pips, of either 15-kcps or 150-kcps frequency. The calibration pips 
(positive) are passed through a shaping circuit consisting of a cathode-follower 
clipper (Vi), Either of these sets of signals (selected by the "clearing switch") 
is applied to the control grid of V 3 . The screen of V 3 is fed in the usual way by 
a droppingresistor and bypass condenser. The suppressor grid of Vg is normally 
at - 80v (thereby cutting the tube off) but is gated to Ov by positive pulses corres- 
ponding to the BandC strobe initiating pulses (these are obtained from the screen 
circuit of VjjJ A large amount of negative feedback is used (resistance- capaci- 
tance coupling from plate to grid). As a result of this, three significant effects 
are produced: 

(a) At times other than those occupied by the B and C strobes, V 3 is cut off and 
positive pulses (either received pulses or calibration pips) appear at the 
plate via the feedback network. 

(b) During the time occupied by the B and C strobes, Vg conducts, and the sig- 
nals appear inverted at the plate. The amount of feed^ck is pre- adjusted so 
that the amplitude of the output is the same as before. 

(c) During the B and C strobes, the voltage level at the plate of V 3 is lowered 
due to the fact that the tube is now conducting, and this portion of the sweep 
will therefore be lower than the main part. 

Figure 11-06 (page ll.ll)showstheappearanceof the display under various 
conditions . 

The D pulses appear on both traces of the main time-base because their spacing 
is 6 milliseconds whereas the time of one trace (plus return) is 2 milliseconds. The 


GEE 


11.15 


period of the whole display is 4 milliseconds. 

When signals are received, the repetition rate at the transmitter (500 cps) 
may not agree exactly with that at the receiver, causing the received pulses to drift 
sideways. This is corrected by a vernier adjustment to the crystal frequency. When 
the pattern is thus locked, the operator proceeds with pulse alignment and time- marker 
counting as already described. By means of special Gee charts, on which the hyper- 
bolic lines of position for varying numbers of 15-kcps time- interval markers are 
overprinted in color, a fix may be plotted. 

In areas where the A - Band A - C lattices intersect at too acute an angle for 
good accuracy, the D pulses may be used instead of either the B or the C pulses, 
Since the D pulses occur on both traces, the appropriate strobe marker can be used 
as desired. 

The usual arrangements are made to blank out the cathode- ray beam during 
flyback, and also to intensify the beam during the B and C strobe time-base traces. 


Bibliography 




Identification 

Classification 

Title 

Issued bv 

CD 0808 D 

Confidential 

Gee - R ARI 5342 

WDGS 

CD 0895 D 

Confidential 

Type 7000 Station T1365 

WDGS 

JEIA 1342 

Secret 

Evaluation of S.W. Gee chain 

CCDU, RAF 

CD 0895 F 

Confidential 

Type 7000 Station Ground Equipment 

Air Ministry 

CD 0208A (2) 

Confidential 

G-H Airborne equipment Mkl ARI5525 

Air Ministry 

JEIA 7031 

Secret 

Accuracy of Gee 

Coastal Com- 
mand 

SD 0208(2) 

Secret 

Gee Mkll Equipment ARI 5083 

Air Ministry 

WA 116 36 

Confidential 

How Gee works 

Air Ministry 

JEIA 8883 

Secret 

Trials of G-H Mkll 

Intelligence 

Division,UB.N. 

Report 625 

Secret 

The Future of Hyperbolic Navigation 

MIT 





LORAN 


12.01 


Type of System 

Differential range, yielding hyperbolic lines of position. 

Useful Range 

Approximate practical maximum ranges over sea are given in Table 12=01. 
It should be noted that areas adjacent to transmitting stations are areas of low pre- 
cision and in some cases are not usable; also that a craft must be within the ser- 
vice area of each of two Loran pairs in order to obtain a fix. Figures given are for 
the 70-100 kw. transmitters in present use. 


Table 12-01 Maximum range (statute miles) over sea. 



Day 

Night 


Ground Wave 

Sky Wave 

Ground Wave 

Sky Wave 

Standard Loran 

850 


600 

1600 

SS Loran 

- 

- 


1600 

LF Loran 

1500-2000 


All figures in the above table are based on tests made in temperate latitudes. Re- 
ceived noise level is greater in tropical latitudes and less in polar regions, modi- 
fying the figures accordingly. Regular Loran readings have been made (by skywave) 
at distances of over 2000 miles, but such distances cannot be relied upon. Higher 
noise level during night hours accounts for the reduced night-time ground- wave 
ranges. 

Accuracy and Precision 

See discussion on pages 12.09-12.12. 

Presentation or use of Data 

The presentation is visual (pulse matching and time- marker counting on the 
screen of a cathode- ray tube). Proposals have been made for an automatic plotting 
board which would obviate time- marker counting and interpolation of a line of posi- 
tion on the Lor^n chart. A recent modification presents time- differences on a mech- 
anical counter, obviating actual counting of time -markers. 

Operating Skill Required 

(a) Ground Stations: The maintenance of synchronization between "master” 
and "slave" stations (see bel6w) calls for skilled monitoring. The operator should 
know all that the craft navigator knows, and more besides. The crew required at a 
standard Loran station with its own power supply is from 20 to 30 men. 

(b) Craft: The Loran operator must be trained in the use of the specialized 
receiver and indicator, and also in the interpretation of the received pulses display- 
ed. 40 or more hours of instruction are given to operators having a previous Imow- 
ledge of general navigation. 

(c) With ground- wave signals, aline of position is obtainable in approximate- 
ly one minute, a fix in from two to five minutes. With sky-wave signals, the opera- 
tor maintains a continuous watch and obtains lines of position when conditions are 
favorable . 

Equipment Required 

(a) Ground: A chain of Loran stations, providing the two sets of hjrperbolic 
position lines necessary for a fix, may consist of four stations, of which two are 
masters and two slaves. In the case of standard Loran, one master may control two 
slaves, or one slave may be pulsed by two masters, making a total of three trans- 
mitters, each of which will be of 70-100 kw. peak power. Mast or inverted-L anten- 


12.02 


LORAN 


nas are used. Receivers and monitoring equipment are also required, and (in the 
case of slave stations) synchronizing equipment. 

(b) Craft: A specialized Loran receiver and indicator are used, weighing 
about 70 lbs. (Airborne equipment). 

Radio Frequency Spectrum Allotments Required 

Standard and Sd Loran (see page 12.08) use frequencies in the range 1700-2000 
kcps (wavelength 176-150 meters). For LF Loran, a frequency of 180 kcps is pro- 
posed (1667 meters). Since the transmission consists of pulses, the bandwidth re- 
quired may be given a nominal value of 50 to 70 kcps for Standard and SS Loran, 10 
kcps for LF Loran. Channel space is however economized by having several pairs 
of stations transmit on the same frequency, using different pulse repetition rates. 
The optimum bandwidthfor the receiver is stated to be about 50 kcps (at 6 db. down). 

Present Status of Development 

Standard Loran is a well-established long-distance navigational aid. It is the 
only long-distance radio navigational aid available at the present time over large 
areas of the Atlantic and Pacific. 

SS Loran, originally proposed as a night- bombing aid over Germany, is well 
established but not so widely used as Standard Loran. 

L.F. Loran is expected to be in operation in the Pacific in the fall or winter 
of 1945. 

General Principles of Operation 

The "master” station of a Loran pair transmits pulses (of about 40 micro- 
seconds duration for Standard Loran, or 300 microseconds for LF Loran) at a repeti- 
tion rate which is characteristic of the particular pair. For most Standard Loran 
stations this is near 25 pps, for SS Loran near 33 1/3 pps. Pulses from the 
master station are received at the craft after an interval representing the time taken 
for the transmission to travel the distance from master station to craft. Pulses from 
the master station are also received at the "slave" station after some other interval 
which is characteristic of a given pair of stations and is proportional to the base- 
line used (distance from master to slave). These received pulses cause the slave 
station to transmit pulses of its own at the same repetition rate. A fixed delay time 
is introduced at the slave station between received and transmitted pulses, for 
reasons which will be explained below. The slave transmission is thus synchronized 
with, or locked to, the master transmission. This is the significance of the terms 
"master "and "slave". Pulses from the slave station arrive at the craft after a time 
interval representing the distance from slave to craft. The craft therefore receives 
two series of pulses, one from master and one from slave. The time interval between 
the arrival of master and slave pulses is measured at the craft by means of the in- 
dicator on which the received pulses, and also suitable time- marker pips, are dis- 
played. From the discussion of hyperbolic systems given in section 1, it will be 
seen that the time delay between received master and slave pulses is characteristic 
of a hyperbolic line of position, and that two intersecting lines (obtained from dif- 
ferent Loran pairs) yield a fix. 

The Loran indicator makes use of a cathode- ray tube, on which appear two 
linear time-base sweeps, swept in succession and displayed one under the other. 
If the prf is 25 per second, then one master pulse and one slave pulse will be dis- 
played every 40,000 microseconds. Now the measurement process requires that of 
these two pulses, one appear on the upper part of the sweep and the other on the 
lower part. Therefore, the slave pulse should be delayed by at least one-half of the 


LORAN 


12.03 


repetition period, or 20,000 microseconds, to ensure this result. Furthermore, in 
order to accomodate the time differences to be encountered at extreme parts of the 
desired coverage area, an additional "coding" delay is introduced at the slave sta- 
tion. These delays in the emission of the slave pulses constitute the fixed time de- 
lays mentioned above. 

Theseandother time intervals are represented in a system of symbols which 
has become standard terminology in Loran. They are as follows: 

c = velocity of radio propagation (= 186,200 statute miles per second, or 0.1862 miles 
per microsecond) 

T = indicated time difference 
T' = true time difference 
L = recurrence interval 
D = absolute delay 
8 = coding delay 

j8 = time taken for pulse to travel from master to slave 
All times are measured in microseconds. 

Consider the distance- time relations for the locations of master, slave and 
craft shown in Figure 12-01. Since the velocity of propagation of radio waves may 
be taken as constant (186,200 miles per sec.), it is convenient to measure any dis- 
tance in terms of the time (in microseconds) required for transmission over that 
distance. Thus the base-line AB is /3 microseconds long, where /3 = AB/c. Assume 
now that a pulse is radiated from the master station A at some arbitrary time t = o. 
This pulse will arrive at slave B at a later time t = /3, and the slave pulse will be 
radiated from B at a still later time t = P+L/ 2 + S . The absolute delay D between 
the emission of master and slave pulses is therefore D = + l /2 + 8 

If the craft were situated at P, the distances AP and BP being equal, the true 
time difference between received pulses will also be j3 + l /2 + S . This will be the case 
at any point on the line PP ' which is the perpendicular bisector of AB. However, due 
to the fact that a time difference of l /2 is automatically taken care of by the presenta- 
tion of the two pulses on the upper and lower parts of the linear time base, the low- 
er trace of which starts exactly l /2 microseconds after the start of the upper trace, 
the indicated time difference as read by the navigator anywhere on PP' will be /3 +5 

Suppose that RR' is a hyperbola such that the difference between the slave 
and master distances to any point on it (BR - AR) is constant and equal to x. Then 
the true and indicated time differences for this line will be T' = jS + l /2 + 8 +x/c , 
andT = p + 8 +x/c . Likewisefor the hyperbola QQ* , on which x' = (BQ - AQ), the time 
differences are T' = p + l/z+ 8 +x'/c and T = p + S +x'/c . Here x'will be numerically a 
negative quantity. The extensions of the baseline, which are the limiting hyperbolae, 
correspond to indicated time differences of S and 2p + 8 as shown on Figure 12-01. 
It follows that a set of hyperbolae can be drawn on a chart, and can be marked with 
the indicated time differences (allowing for slave station coding delay) they repre- 
sent. Thus the navigator measures his indicated time difference and obtains a line 
of position, by interpolation between the printed hyperbolae if necessary. 

Disposition of Stations, Coverage Areas 

(a) Standard Loran: In Standard Loran, the maximum practical base-line 
length is about 600 miles. This is so because reliable ground- wave reception, with 
adequate signal- to- noise ratio for synchronization of slave to master, is not obtain- 
able over much larger distances than this. A 300- mile base line is conventional and 
gives ^ = 1611 microseconds. S is customarily fixed at 1000 microseconds. This 


12.04 


LORAN 



gives maximum and minimum indicated time differences of 1000 microseconds and 
4222 microseconds. Charts are prepared with hyperbolae marked every 100 micro- 
seconds. A set of hyperbolic position lines is shown in Figure 12-02, and two sets, 
using a master station and two slaves with base-lines intersecting at 135°, in Figure 
12-03. With the latter arrangement, or with a straight-line chain, the coverage area 
for a fix will be determined by the limit at which ground- wave or sky-wave reception 
from all three stations is obtainable. Sky-wave reception at these frequencies can 
be relied upon only at night, and then should not be used at distances less than 300 
miles. The outer limit for sky-wave reception is (as already stated) about 1600 miles. 
Figure 12-04 (a) shows the ground-wave service area of a 3-station chain. The full 
lines indicate the area within which a line of position may be obtained from master 
A and slave B. A fix may then be obtained within the shaded area. A base-line of 
300 miles is assumed (600 miles from B to C). Figure 12-04 (b) is drawn to one- 
half of the scale of (a), and shows the sky-wave coverage (shaded area). The dotted 
lines in both figures show the boundaries of areas adjacent to the base-line within 


LORAN 


12.05 




Fig. 12-02 Loranpair 


12.06 


LORAN 



Fig. 12-03 Loran triplet 


which the "geometrical precision "is poor (lower than 1 mile/microsecond for ground- 
wave) for one or other of the pairs. Geometrical precision is further discussed on 
page 12.10. 

It will be seen that a chain of Loran stations will have a useful sea coverage 
area which is roughly in the form of a strip extending parallel to the coast line on 
which the stations are located. 

(b) SS Loran: Other factors being constant, the greatest precision of fix will 
be obtained in areas where Loran hyperbolae intersect at right angles. With Standard 
Loran, such areas are localized and are small in extent. The longer the base-lines 
used, the more extensive will these areas of high precision be. This leads to the use 
of sky-wave transmission for slave synchronization, the base-line between master 


LORAN 


12.07 



(a) Ground wave 



(b) Sky wave 


Fig. 12-04 Standard Loran coverage areas 


12.08 


LORAN 



Fig. 12-05 SS Loran coverage area 


and slave being extended to 1300 miles or more. With SS (Sky-wave synchronized) 
Loran, the two pairs are located so as to straddle the required coverage area, their 
base lines being approximately at right angles. This gives a large area in which 
the Loran hyperbolae are approximately straight and parallel lines, the two sets in- 
tersecting nearly at right-angles. 

Figure 12-05 shows the coverage area for 1500-mile base-lines and station 
locations as shown. Ground waves are not normally used with SS Loran, since this 
would usually involve cross- matching (one ground wave and one sky wave). Areas 
within 300 miles of any of the stations are therefore not represented as covered. 

(c) LF Loran: Using frequencies of the order of 180 kcps, ground- wave 
propagation may be relied upon to distances of about 1 500 to 2000 miles with the 
transmitter power contemplated. This means that longer base-lines may be used, 
with ground- wave service areas and precision comparable to those given by SS Loran, 
but without the disadvantages attendant on the use of sky waves. Longer pulses (300 
microseconds) and very large antennas (625 feet, umbrella loaded) are proposed. 
Since transmission at these frequencies must of necessity involve the use of band- 
widths narrower than 50 kcps, the rise time of the pulses will be longer than is the 
case with standard Loran practice. This means that the pulse- matching technique 
used must take rather carefully into account the details of pulse form, since it is not 
usually possible to match amplitudes in order that similar portions of the leading 
edges of pulses will have the same slope. The reason for this is that since long pul- 
ses are used, there is the danger that sky-wave returns, mixing in variable phase and 
amplitude with ground- wave returns, will distort all but the initial portion of the ris- 
ing pulse-front. Since this initial portion is of rather gradual slope, a limitation is 
thus imposed on the precision of match attainable. 


LORAN 


12.09 


A technique of "Cycle matching "has therefore been proposed'for possible use 
in s la ve= station s 5 mchronization, and perhaps ultimately for use in the craft. Indi- 
vidual RF cycles at corresponding positions in the two pulse envelopes would be se- 
lected and used for comparison. This refinement is still in the experimental stage. 
A further discussion of the effect of tuned circuits on pulse rise times is given in 
section 1 . 

Pulse Rates 

Since the received pulses are displayed on a time-base on a CRO screen, the 
recurrence rate of the time-base must be synchronized to that of the pulses if a sta- 
tionary display is to be obtained. Since it must be made possible to receive several 
pairs of stations to give fix coverage over different areas, two possible methods of 
accomodating these various channels exist: 

(a) Different radio frequency for each pair, same prf for all stations. 

(b) Same radio frequency for all stations, different prf for each pair. 

With pulse transmissions at this frequency, the spectrum allotments required would 
be prohibitive if scheme (a) were used. Loran therefore uses scheme (b) allowing 
several pairs of stations to be "stacked" at the same frequency. 

Provided that the pulses to be observed stand still, the operator will not be 
unduly distracted if the rate of drift of other pulses (to whose prf he is not synchron- 
ized) across the CRO screen is quite slow. The prf' s ("rates") used need not there- 
fore differ by large amounts. Adjacent pulse repetition periods differ by 100 micro- 
seconds. Corresponding prf's, periods and "rates" are as follows: 




prf (per second) 


25 


(approximate) 
period (microseconds) 
"rate" (station pair) 


40,000 39,900 39,800 39,700 etc. up to 39,300 

0 12 3 etc. up to 7 


A similar group of pulse repetition frequencies based on 33 y pps is also used. The 
navigator has on the indicator panel a multi-position switch, which selects the rate 
desired; that is, causes the received pulses from the two stations corresponding to 
this rate to remain stationary on the screen, while any pulses which may be received 
on other rates drift by. 

Uncertainties in Loran Line of Position 

Various factors combine to produce uncertainties in a line of position, ex- 
pressed as the probable error of a numerical indicated time difference. These fac- 
tors may be classed in two groups: 

(a) Systematic. This includes all effects traceable to the layout of the sys- 
tem itself. 

(b) Operational. This includes human factors, both at the craft and in slave- 
station synchronization, and effects which cannot be predicted, such as variations in 
noise level and in ionospheric conditions. Some notes on various factors follow: 

1. Errors in measurement of time intervals with the Loran indicator. 

It has been estimated, on the basis of large numbers of observations made 
under average conditions, that the probable error of a Standard Loran time measure- 
ment is about 1 microsecond. This means that out of a large number of measure- 
ments, 50 per cent will depart from the true value by less than 1 microsecond and 
50 per cent by more than 1 microsecond. Assuming a standard error curve distri- 


12.10 


LORAN 


bution, another way of stating this result is to say that 90 per cent of the measure- 
ments will be in error by less than 2.44 microseconds and 10 per cent by more than 
this amount. These figures are obtained from many measurements. Under good 
conditions and with a skilled observer, readings in error by only 0.5 microsecond 
have been consistently obtained. It is clear that no one figure for this error will be 
acceptable to all interested persons. 

2. Errors in synchronization. 

The slave station operator has essentially the same problem as the craft na- 
vigator. Master and slave pulses are displayed on his monitoring oscilloscope, and 
he must maintain the correct time delay between them. With SS Loran, there will 
be periods when reliable observations are impossible owing to varying ionospheric 
conditions. Accordingly, the timing oscillator at slave stations must be so care- 
fully stabilized as to be capable of running free during these periods with negligible 
drift. However, errors in synchronization have been known to occur. 

3. Resultant error in a line of position. 

The effects of the two errors just mentioned are to broaden Loran hyper- 
bolae into bands, and to change the location of the hyperbolae slightly. The resul- 
tant error in line of position varies with the location of the craft, since the hyper- 
bolae spread further apart as the distance from the stations increases. The error 
in line of position is therefore the product of two quantities: 

(a) The net timing error in microseconds. 

(b) The change in position per unit change in time difference, measured in 
miles per microsecond, at the point in question. 

The first of these quantities is statistical and has already been discussed; the se- 
cond is purely geometrical. For example, two hyperbolae, corresponding to time 
differences which are 1 microsecond apart, cut the base line at points which are 
separatedby 0.093 mile, or 492 feet. Thus, on the base line a timing uncertainty of 
2.44 microseconds corresponds to a positional uncertainty of 2.44 x 492 or 1200 feet 
(0.23 miles). This is the minimum positional uncertainty for a timing uncertainty of 
2.44 microseconds. At points far distant from the base line, the positional uncertainty 
may be several miles. 

Regarding the second, or geometrical, quantity, it may be shown that the 
number of miles error per microsecond error is constant for all points on an arc 
of a circle which passes through the locations of master and slave stations. A fam- 
ily of such circles (of which the base line and the extensions of the base line are the 
limiting cases) is shown in Figure 12-06. Portions of the 1 mile/ microsecond arc 
were drawn in Figure 12-02 (a), and these define the wedge-shaped areas adjacent 
to the base line extensions in which poor precision is obtained. It should be pointed 
out that the error in position per microsecond error for the circles in Figure 12-06 
is measured normal to the hyperbolic position lines and not along the circles them- 
selves. It will be realized from Figure 12-06 that the longer the base line is, the 
larger will be the area over which a given precision is atUinable. SS Loran makes 
use of this fact. 

4. Error in a fix. 

Since a fix is determined by the intersection of two hyperbolae, and since 
uncertainty exists as to the location of each hyperbola, the actual position of the 
craft is somewhere within an area whose boundaries are the limits of uncertainty 
of the two hyperbolae. The shape, area and dimensions of this figure of uncertainty 
dependon the angle of cut between the h^erbolae, and this makes quotation of numer- 
ical values for error in a fix impossible. The subject is discussed further in sec- 
tion 1 . 


LORAN 


12.11 


f 


\ 





Fig. 12-06 Circles of constant miles per microsecond error 


12.12 


LORAN 


5. Errors in assumed values of base-line length and velocity of propagation. 

These factors amount to a change in the orientation and location of the Loran 
hyperbolae. Their effect is further discussed in section 1. 

6. Skywave errors. 

If skywave measurements are made with Standard Loran, a skywave correction 
must be applied, due to the fact that the difference between the skywave propagation 
paths to the craft is not the same as that between ground wave paths unless the craft 
happens to be on the perpendicular bisector of the base line. This correction is 
printed on the appropriate parts of Loran charts. It varies with position, being large 
at points which are close to one transmitter of a pair but far from the other. Since 
the skywave delay curve flattens out to a constant value for large distances (see 
Section 1), the skywave correction (which is equal to the difference between the two 
skywave delay values) tends toward zero as the distances of the craft from the two 
transmitters increase. Since the height of the E-layer of the ionosphere is not cons- 
tant, the skywave delays for the two transmission paths involved, and therefore the 
skywave correction, are subject to changes. These changes introduce random errors, 
so that the probable error of a skywave reading is several times that of a ground- 
wave reading. 

With SS Loran, where skywaves are used both for position determination and 
also for slave station synchronization, such errors partially cancel. A more extend- 
ed discussion of skywave propagation and resultant errors is given in section 1 . 
One of the merits of SS Loran, however, is that random errors due to changes in 
propagation conditions are somewhat offset by the excellent angle of cut obtainable 
over most of the service area, and by the greater area over which a given geometri- 
cal precision is attained, due to the long base-lines. 

7. Errors due to incorrect interpretation of received pulses. 

When skywaves are used, the received signal is not a single pulse, but con- 
sists of a train of pulses reflected from various layers of the ionosphere. For Loran 
purposes, the pulse received by single- hop transmission from the E- layer of the 
ionosphere, known as the Ej pulse, is chosen. If a ground- wave pulse is visible, the E^ 
pulse will be the first one following it. Since the intensity of reflection and the degree 
and nature of the polarization of the various components of the signal are non- related 
functions of time, the observer sees a constantly changing picture. A reliable fix 
using skywaves thus demands an experienced operator who can recognize "normal" 
conditions and take a measurement when he sees that these conditions exist. If a 
ground wave pulse is used by mistake for an Ej pulse, large errors are introduced 
and these are usually detectable by their inconsistency with other data at hand. 

The Loran Receiver and Indicator 

Several different models of Loran craft equipment are in use. The differences 
between them are in general differences in detail only. In one model (the DDE, in 
production) the time-difference is shown on a direct- reading mechanical counter 
driven by the knobs used for pulse alignment, and time- markers on the CRO pattern 
are therefore not provided. In another model (AN/APN 9), the fast sweep is logarith- 
mic and a three- inch cathode- ray tube is used. The descriptions which follow are 
concerned with the AN/APN 4 equipment. 

Receiver (R - 9/APN 4) 

Figure 12-07 shows a block diagram of the receiver. The R.F. amplifier is 
tunable to four frequencies, selected by a four-position switch. These are 1750, 
1850, 1900 and 1950 kcps in model R-9B/APN4; and 1850, 1900, 1950 kcps and 9600 
kcps (no longer used) in model R-9A/ APN4. Three wave- traps are provided, tuned 
to the intermediate frequency which is 1050 kcps. The bandwidth of the IF amplifier 


LORAN 


12.13 



Fig. 12-07 Block diagram -- Receiver 


is 45-60 kcps. The gain control is variable cathode-biasing applied to the RF ampli- 
fier, converter and first two IF stages. The gain of the 3rd IF stage is gated in syn- 
chronism with the two parts of the indicator display, so that different degrees of gain 
maybe usedfor master and slave pulses in order that pulse amplitudes may be match- 
ed on the display. The detector stage is a conventional diode with resistance-capaci- 
tance load except that the input is to the diode cathode rather than to its plate. A high- 
pass filter, which may be switched out, is provided between detector and video ampli- 
fier. This is useful for the suppression of low-frequency components of noise. The 
video amplifier consists of one (triode) stage of amplification, which gives limiting 
action and feeds a cathode follower. A clamping diode is used at the grid of the cathode 
follower. The DC power- supply for both receiver and indicator is at 260v, voltage- 
regulated. 

Indicator ID-6B/APN4 

General Principles 

Figure 12- 08 shows a block diagram of the indicator circuits. Power supplies 
are not shown. A linear sawtooth time- base voltage is applied to the horizontal de- 
flection plates of the C.R. tube. The sweep- generator is of the triggered type, so 
that different sweep speeds may be used. Video signals from the receiver, or alter- 
natively time markers, are applied to one of the vertical deflection plates, and a 
square- wave trace- separation voltage to the other. Since the repetition rate of the 
entire display is 25 cps (on station- rate 0), the square wave must have this basic 
frequency. Video pulses from the receiver, pedestal voltages and 10- microsecond 
markers deflect the cathode- ray beam upwards; 50-microsecond and 500-micro- 
second markers produce downward deflections, and 2500- microsecond markers de- 
flect the beam both above and below the time base. 

All timing operations are controlled by a 100-kcps crystal oscillator. This 
is of the tuned-plate tuned-grid t 3 q)e, and its frequency is variable by ± 35 cps by 
means of two small trimming condensers. One of these constitutes the framing 
control: if this condenser is adjusted so that the received pulses are stationary on 
the display, the oscillator must then be in exact synchronism with the accurately 
controlled crystal at the transmitter, and the various time markers will be accurate- 
ly positioned. The other trimming condenser allows the pxilses to be shifted to right 
or left on the display (see Left- Right circuits, page 12.20). 

Six counters follow for frequency division. The sixth counter is arranged to 
divide by either 3 or 4 (High or Low prf ) to accomodate the two basic groups of repe- 
tition rates used, based on 25 and 33 cps frequency. The frequencies and repetition 
periods of the pulses at various points in this divider chain are as shown in Figure 
12-08. Signals from the crystal and from the outputs of the first and third counters 
are used to form the 10- microsecond, 50- microsecond and 500- microsecond time 
markers by way of suitable pulse- shaping circuits. 2500- microsecond time markers 
are obtained from the output of the fourth counter. 



STATION SELEC. 50 C/S 

AND -' ■■■ 

LEFT- RIGHT 20,000 pS 


LORAN 


12.14 



Fig. 12-08 Block diagram, Indicator and timing circuits 




LORAN 


12.15 


Time- difference measurement is accomplished by bringing the pulses on the 
upper (A) and lower (B) traces into alignment, and then counting time markers to 
obtain the time interval through which the lower pulse (B) was shifted to attain this 
result. For this purpose two pedestals are used. These are raised sections of the 
main sweep traces and are obtained by applying rectangular pulses to the lower CRO 
deflection plate. The A pedestal is delayed after the beginning of the main A sweep 
by a fixed amount (2500 microseconds). The corresponding video pulse is made to 
occupy a position on the A pedestal by momentarily changing the frequency of the 
sweep, thus causing the pulse to drift across the display: at the same time the B 
video pulse will of course move in the same direction on the lower trace of the dis- 
play and by the same amount. This is accomplished by means of the Left- Right switch, 
whose action will be described later. The B pedestal is delayed after the start 
of the B sweep by an amount which is variable by coarse and fine controls: thus, 
by the use of these controls, the B pedestal may be brought under the B video pulse. 

Switching to a faster sweep speed, the two sweeps are of shorter duration 
and are now initiated and terminated by the leading and trailing edges of the rectangu- 
lar pulses which previously formed the pedestals. This amounts to spreading out the 
time scale until the pedestals themselves are the sweeps. The A and B pulses are now 
accurately aligned by further use of the B pedestal fine delay control. The exact 
sequence of operations, including counting, is given later. 

As noted above, the A pedestal delay is fixed at 2500 microseconds. The B 
pedestal delay is variable from 1700 to 13,200 microseconds, the coarse control 
giving delays of 1500 to 12,500 microseconds in steps of 500 microseconds, and the 
fine control being continuously variable from 200 to 700 microseconds. The dura- 
tion of the pedestals is 250 or 750 microseconds, controlled by the setting of the 
sweep- speed switch. 

The six counters are of the step- counter blocking- oscillator tjrpe with grid 
bias applied to the counter tube. A condenser in the grid- circuit is charged in steps 
by pulses fed in from the preceding counter. The counter tube fires when its grid 
condenser has acquired a definite number of these "step" charges, the number de- 
pending on the bias and circuit constants used. Thus the first counter fires every 
five cycles of the 100 kcps pulses, the second counter fires every other time 
the first counter fires, and so forth. To secure the correct prf for the received 
pulses to be used, feedback is taken from the output of the sixth counter to either the 
second or third counter grid condensers, or to both. Likewise in order to move the 
video pulses to right or left on the display, the feedback arrangements may be tem- 
porarily altered. These processes and the circuits connected with them will now be 
explained. No two counters have identical circuits, but all counters operate on the 
same principle. The first and second counter circuits are shown here. 

Figure 12-09 shows the circuit of the first counter. The 100 kcps crystal 
oscillator is followed by a limiter stage (not shown), in which a triode tube is driven 
from cut-off to saturation. The resulting voltage waveform as shown is applied at 
A to a cathode-follower buffer stage Ti, which drives the blocking- oscillator counter 
tubeT 2 . The positive bias and the time constant in the grid circuit are so arranged 
that T 2 fires on every fifth pulse from T^. The output at the plate consists of sharp 
negative and positive pulses as shown at B. 

The circuit of the second counter is shown in Figure 12-10. The diode T^ 
clips the negative pulses, and T 2 transmits the positive pulses. The 330 con- 

denser is charged by steps, and the bias on T 3 is so adjusted that this tube fires on 



the second step. When the tube fires, the 3Z0 condenser is discharged via the 
grid circuit and the cycle repeats. The output to the third counter therefore con- 
sists of sharp negative and positive pulses as shown. 

It will be observea that the 330 yyi. condenser may also be charged (or dis- 
charged) by feedback from the sixth counter. If an auxiliary charge of one step is 
applied in this fashion, the result will be to eliminate one 50- microsecond interval 
from the cycle of operations every time the sixth counter fires. This process is 
further discussed under the heading of station selection. 

The remaining counters differ mainly in the sizes of the components used, 
and in the exact method of obtaining bias for the counter tube. The fifth and sixth 
counters are coupled by a tuned circuit which "rings" when pulsed, and the sixth 
counter is actually triggered by the first positive peak of the damped oscillation 
thus produced. This introduces a constant delay of about 25 microseconds in the 
firing of the sixth counter tube. The reason for this is to ensure that the auxiliary 
step charges, placed on the second and third counter condensers when feedback is 
used, shall not coincide with normal step charges from the preceding counter stages. 
The sixth counter also has two possible values of grid bias, to allow division by 3 
(output, pulses of 15,000-microsecond repetition period) or by 4 (output, pulses of 
20,000- microsecond repetition period) for the 33 l/3cps or 25cps repetition rates. 

Figure 12-11 shows the station selector and left- right circuits. It will be 


LORAN 


12.17 


+ 260V. 



recalled that Loran pairs operate on the same radio frequency but with different 
pulse repetition rates which are as follows: 


Rate (station) 

p.r.f. 

Rate (station) 

p.r.f. 

0 

25 cps 

4 

25-^ cps 

1 

1 

25r5 cps 

5 

OK 5 

25 cps 

2 

25-^ cps 

6 

3 

25-^ cps 

3 

3 

25ig cps 

7 

25^ cps 


A similar group ofprf's, based on 33 cps, maybe used by allowing the sixth 
counter to divide by 3 instead of by 4 (prf switch). In the explanation that follows, 
only the 25 cps group is considered, for the sake of simplicity. 


Station (Rate) Selection ^ 

Consider the change of frequency required in switching from rate 0 to rate 
1. If this were done by changing the crystal oscillator frequency, the necessary 
change would be 250 cps. This would introduce an error in the spacing of all the 
time markers, and in addition, would seriously reduce the amplitude of the crystal 
output. The change must therefore be made in the counter operations rather than 
in the crystal frequency. 

Referring to Figure 12-11, pulses from the 6th counter may be fed back to 
either the second or third counters or to both. On rate 0 there is no feedback and 
the prf is 25 cps. On rate 1, feedback is to the second counter only by way of 


12.18 


LORAN 



Fig. 12-11 Station and Left-Right circuits 

Ganged switches: sweep speed: Si, S4, S7 
station: S3, Se, Sg, Sg 
Left-Right: S2, S5 

C4 , S5, C3, S3, and the diodes T2 and T3 . Negative pulses are clipped by T2 
and positive pulses transmitted by T3. The magnitude of the feedback pulse is con- 
trolled by C3, and is such that the firing of the second counter is advanced one step. 
On rate 2, there is no feedback to the second counter (S3 being then grounded) but 
feedback is applied to the third counter via C4 , Ciq and Cn in parallel, Sg and the 
diodes T4 and Tg. clips negative pulses and Tg transmits positive pulses. The 
magnitude of the feedback is such as to charge the third counter condenser by one 
step, being controlled by the adjustment of which is critical. 

On rate 3, one-step feedback is applied to the second counter by way of C3 
and S3, and at the same time one- step feedback is still applied to the third counter 
since positions 2 and 3 on Sg are connected. The feedback to the third counter is 
slightly augmented by C12 (which is in parallel with Ciq and Cn for position 3 of 
Sg and Sg) to allow for the extra loading on the feedback source imposed by the sec- 
ond and third counter circuits simultaneously absorbing pulse energy. 

On rate 4, no feedback is applied to the second counter, but two-step feedback 
is applied to the third counter via C^, Cg and Cg in parallel, Sg, T^ and T5. The 
necessary two-step magnitude is adjusted by Og , the combined capacitance 01 (Jg 
and Cg being greater than that of Cio and Cii . Proceeding in this fashion, the appli- 
cation of feedback to the second and third counters for the eight "station" switch 
positions is as follows: 


LORAN 


12.19 


Station 

switch 

position 

Feedback to 

Second Counter 

^ Feedback to 
Third Counter 

0 

none 

none 

1 

one step 

none 

2 

none 

one step 

3 

one step 

one step 

4 

none 

two steps 

5 

one step 

two steps 

6 

none 

three steps 

7 

one step 

three steps 


Considering now the effects produced, it will be seen that feedback is applied once 
during each sixth- counter cycle. One-step feedback to the second counter causes 
this and all succeeding counters to fire 50 microseconds earlier than would other- 
wise have been the case, and therefore shortens the sixth counter period by this 
amount. Since the square-wave generator which determines the frequency of the dis- 
play is in effect a 2:1 frequency divider, the change in the display period (prf) will 
be 100 microseconds (twice that of the sixth counter period). 

One-step feedback to the third counter changes the sixth counter period by 
100 microseconds, and the prf by 200 microseconds. The overall effect of the 
feedback in various switch positions is therefore as follows: 


Station 

switch 

position 

Change in Sixth 
Counter Period 

Sixth 

Counter 

Period 

Display 

Repetition 

Period 

0 

0 

20,000 Microseconds 

40,000 Microseconds 

1 

50 Microseconds 

19,950 

39,900 " 

2 

100 

19,900 

39,800 " 

3 

150 

19,850 

39,700 

4 

200 

19,800 

39,600 

5 

250 

19,750 

39,500 

6 

300 

19,700 

39,400 

7 

350 

19,650 

39,300 




12.20 


LORAN 


Operation of the Left- Right Switch 

Assuming that the crystal fine frequency control has been adjusted to give a 
stationary pulse display, it is still necessary to be able to move the pulses on to their 
respective pedestals. This is done by momentarily changing the prf of the display, 
causing the pulses to drift across it. Now if this were to be done by changing the 
crystal frequency, the change required with slow sweep speeds would be inconvenient- 
ly large for a reasonably rapid' rate of drift of the pulses on the display. It is there- 
fore done bothby a slight change in crystal frequency (effective at fast sweepspeeds) 
and by applying either positive or negative feedback (in addition to any feedback 
used for station selection) from the sixth counter to the second and third counters, 
effective at slow sweep speeds only. 


Referring to Figure 12-11, the cathode of is normally at +92 volts (due to 
Rl and R3) which is sufficient to maintain the tube in a non-conducting condition. If 
S2 is moved to the "left" position, pulses from the sixth counter are applied to the 
cathode of via Cj^, and the tube therefore conducts during negative pulses. Each 
timeT^ conducts, the second counter condenser is discharged through it, so that an 
extra 50 microsecond interval is added to the sixth counter period. This will cause 
the frequency of the sweep to be lower, and the pulses will drift to the left on the 
display. R2 serves to prevent the accumulation of d-c charge on Cj, also being so 
large that negligible pulse voltage appears across Ro when S2 is at normal or in 
the "right" position. Si places the cathode of Ti at+260v in sweep- speed positions 
2 through 7 so that the only adjustment of the pulse position possible on these speeds 
is by slight changes in crystal oscillator frequency as noted above. 


In order to move the pulses to the right on the display, the prf must be 
temporarily raised. This is accomplished by feeding pulses from the sixth counter 
through S5 ("right" position), 8,^ (positions 1 or 8), and Sg to either the second or 
third counters, depending on the position of 85. It should be noted that 8g and 8r7 
are not ganged together. Thus in station positions 0, 2, 4, 6, the second counter 
(which is not used for normal "station" feedback in these positions) is accelerated, 
whereas in positions 1, 3, 5 and 7 the third counter (already in use for "station" feed- 
back in positions 3, 5, 7) is accelerated by an extra step, any feedback applied to the 
second counter for "station" purposes being thereby transferred to the third counter. 
This is possible in the case of the third counter because it is a five-step counter, 
whereas the second counter uses only two steps. 87 disables the feedback right- shift- 
ing function for all sweep-speeds except 1 and 8. On the fast sweep-speeds (2, 3, 4, 
5, and 6) left- right motion is performed only by changes in crystal oscillator frequency 
as already noted. Thus the function of the left- right switch comprises two operations: 
(1) Changes in crystal frequency (operative on all sweep-speeds, but effective on 
speeds 2through7 only). (2) Changes in feedback arrangements (operative on speeds 
1 and 8 only). The descriptions which follow assume that the L-R switch is in the 
neutral position, and the station selector switch in position 0 (prf 25 cps). 


8quare-wave Generator 

The square-wave generator is an Eccles-Jordan trigger circuit triggered 
in the cathode circuit by the output from the sixth counter, the positive pulses of 
which have been clipped off. > f 


Pedestal Delay Multivibrators 

The A-pedestal delay is a multivibrator, shown in Figure 12-12. The output 
aphed to Ti, which with its associated elements clips the 
pulses to the grid of To where thev are mixed 
with the differentiated output of the square- wave gLerator 


T2 and T3 form a one-shot multivibrator. T3 is normally conducting, and 


LORAN 


12.21 



Fig. 12-12 A- pedestal delay multivibrator 

T 2 cut off, due to the fact that the grid of T 3 is returned to +260v and that of T 2 to 
only +35v, together with the fact that both tubes share a common cathode resistor. 
Positive pulses from the square- wave generator trigger the circuit, causing T 2 to 
conduct and T 3 to be cut off. The next negative fourth- counter pulse triggers the 
circuit in the reverse direction, the grid- circuit time- constant of T 3 being so chosen 
that T 3 is nearly conducting of its own accord at this time. Further negative pulses 
have no effect since T 2 is now again cut off, and the circuit is ready for the next 
positive square- wave generator pulse. The output at the plate of T« therefore con- 
sists of rectangular positive pulses, of width 2500 microseconds. The width of the 
pulses is accurately controlled by the period of the fourth counter, and their recur- 
rence rate by that of the square- wave generator. The B- pedestal coarse delay is a 
one-shot multivibrator similar to that just described, except that the inputs are 
from the opposite phase of the square- wave generator and the third counter respec- 
tively, and that the grid return of the first triode section is to a point of variable 
positive potential. As this potential is varied, the trailing edge of the multivibrator 
pulse coincides in turn with any one of a sequence of third counter pulses. The width 
of the output pulse is therefore variable in steps of 500 microseconds (on rate 0). 

The B-pedestal fine delay is likewise a one-shot multivibrator, but in this 
case there is only one input (the inverted and differentiated output of the B-pedestal 
coarse delay multivibrator) and the fine delay is therefore continuously variable 
since the termination of the output pulse depends only on the time- constant and grid- 
bias voltage used. Note that the leading edge of the fine delay output pulse is locked 
to the trailing edge of the coarse delay pulse. The total delay is therefore the sum 
of the fine and coarse delays. 

Pedestal Generator 

This is a one-shot multivibrator similar to the preceding three. The outputs 


12.22 


LORAN 


I 

_r 

I 

_k 

I 


square-wave 
(phose A) 

differentiated 



4th counter 
output 

clipped 


A-pedestal 
delay m-v 
output 


square -wove 
(phase B) 

differentiated 


H-H+H-H-H+H- Xr- 

I I 

nrTTTTTTTTTTTT 


I fB-pedestal j 

J J coarse delay I 

j [m-v output J 

1 inverted 


_r 


differentiated 

i B-pedestal 
fine delay 
m-v output 
A-and B- 
pedestal delays 

differentiated 


I 


inverted 
and clipped 

A and B 

pedestals 

(750psec) 

sweep circuit 
output 


sweep inverter 
output 


I A delay 
I 


I 

I 

r ' ' ' ' I ' ' ■ ' I ' ' 

time 0 2500 5000 

(psec) 








1 

!i> t t 

V f V 

1 


1 r 







1 


1 1 

1 1 

1 1 

1 1 1 

1 

^TTTTTl 

rrr 

TTTT 


odjustoble ^ 




in 500 >jse^ 
steps 










t 1 lodjustoble 

1 “TT^0-700;jsec 

i 1 1 

1 

n 

1 1 1 

^ L_i 


1 

1 

nr- 






1 1 

1 ! 

1 


j 1 




>1 



_l/i 



,1 


— 1 — 1 — 

total B delay 

• 

"I 

1 1 

I r 

-1 — 1 — 1 — 1“ 


20,000 22,500 25,000 


Fig. 12-13 Indicator waveforms 


LORAN 


12.23 


from the A-pedestal delay and B-pedestal fine delay circuits are differentiated and 
then inverted by an amplifier stage in which the tube runs at zero bias so that the 
original positive pulses are clipped. The output of this inverter (positive pulses 
corresponding to the negative pulses from the A- and B-fine delay circuits) is the 
input to the pedestal generator multivibrator. The output (pedestals) are therefore 
positive rectangular pulses whose leading edges are delayed alternately 2500 micro- 
seconds after the beginning of the A sweep, and by a variable amount (coarse and fine 
controls) after the beginning of the B sweep. The duration of the pedestals may be 
either 750 microseconds (sweep-speeds 1, 2, 6 , and 7) or 250 microseconds (sweep- 
speeds 3, 4, and 5), controlled by the setting of a switch which changes the grid bias 
applied to the second triode section of the multivibrator. The time relationships 
between these different pulses are shown in Figure 12-13, in which some of the time 
intervals are not drawn to scale for obvious reasons. 

Sweep Voltage Generator 

This circuit is shown in Figure 12-14. It is of the triggered type, and trig- 
gering pulses are applied from either the sixth counter, the fourth counter, or the 
pedestal generator according to the position of the sweep- speed selector switch. 

Referring to Figure 12-14, tubes Tj and T 2 and switch sections Sj, S 2 and 
S 3 are concerned with the selection and shaping of the sweep triggering pulses. In 
positions 1, 7, and 8 of Sj, the outputs of the fourth or sixth counters are clipped by 
Tj^ so that only negative pulses are transmitted to S 2 . S 2 selects either these pulses 
(positions 1, 7, and 8 ) or the output of the pedestal generator. The output from S 2 is 
clamped by T 2 so that the negative voltage excursions at the plate of this tube take 
place from ground potential downwards. These triggering pulses are applied to the 
suppressor grid of T 3 . S3 passes negative blanking pulses to the CRT grid by way 
of C 2 for all switch positions except 8 , and also connects Cj ( 220 /L/tf) in parallel with 
the pulse input, broadening the narrow sweep trigger pulses sufficiently to ensure 
stable action. Somedelay is thereby introduced in the initiation of the sweep voltage, 
but since this is constant it is not objectionable. 



Fig. 12-14 Sweep circuit 


12.24 


LORAN 


Supposing that the suppressor grid of T3 is at ground potential, T3 is con- 
ducting and its plate potential is low. The control grid of T 3 is slightly above ground 
potential and grid current is flowing, due to the fact that the grid is returned through 
a 100,000 ohm resistor to a point at some positive potential provided by the resistor 
network controlled by S4. When a triggering pulse arrives, the s uppressor grid is 
suddenly driven negative and the tube cut off. The plate voltage rises sharply, the 
control grid remaining substantially constant in potential in spite of C3, due to the 
fact that grid current is flowing. This condition will persist as long as T3 is cut off. 
When the suppressor grid is returned to ground potential, the tube again conducts 
and the potential at the plate tends to fall. Due to the coupling action of C3 from 
plate to grid, and to the large amplification of the tube, this fall is relatively slow 
and extremely linear over most of its range, the control grid now being negative. 



OV. 

Suppressor 
Grid of T 3 

A 

ped 


B 

ped 







Plate 
of T 3 


variable. Depending 


L 


750 

or 250 ps 

on B Delay setting ^ 








(b) FAST SWEEP * 


Fig. 12-15 Triggering and sweep voltages 


LORAN 


12.25 


If this condition were allowed to persist, the fall of plate potential would eventually 
flatten out. The rate of fall is controlled by the positive potential to which the grid 
is returned (S 4 and associated voltage -divider networks). The values selected for the 
various positions of S 4 are such that for all sweep speeds the amplitude of potential 
change at the plate of T 3 is the same, and is not sufficient to extend into the non- 
linear portion of the operating range. 

Referring to Figure 12-15, which shows the triggering and output voltages 
for either type of sweep, it is seen that the slow-speed sweeps are initiated by nega- 
tive pulses from the fourth or sixth counters, and that the fast sweeps correspond 
exactly in time with the tops of the pedestals used to produce them (250 or 750 micro- 
seconds). Provision for changing the positive grid return potential of T 3 must be 
made for each different sweep duration, and a section of the pulse repetition rate 
switch accomodates the 25 cps and 33 1/3 cps basic repetition rates. 

T 4 is a sweep inverter tube. By means of a suitable voltage divider and 
negative feedback network, the output from its plate is arranged to be of the same 
amplitude as that from the plate of T 3 , but inverted. 

The twopush-pull voltages so obtained are applied to the horizontal deflection 
plates of the cathode- ray tube through suitable clamping circuits which ensure stabili- 
ty in the horizontal position of the display. Clamping is also applied to the CRT con- 
trol grid, so that the maximum intensity of any part of the display is the same for 
all sweep speeds. The focus and intensity controls of the CRT, and the accelerating 
voltage circuits, are conventional in design. 

Signal- marker Mixer 

Time markers are clipped and shaped, and are mixed with pedestal voltages 
and video signals, in a network which is essentially a four-input vacuum-tube mixing 
circuit, in which the four mixing tubes have a common plate resistor . Suitable switch- 
ing arrangements enable video signals to be eliminated on sweep speeds 5, 6 , 7 and 
8 , and markers on speeds 1, 2, 3 and 4. 

Trace Separation and Amplitude Balance 

For these purposes, outputs from both sides of the square- wave generator 
are applied to a twin cathode-follower. A potentiometer connected between the two 
cathodes of the tube enables a square-wave voltage of adjustable amplitude and either 
sense to be applied to the cathode of the third I.F. amplifier tube for amplitude bal- 
ance as between the A and B pulses. Trace separation vpltages, different in ampli- 
tude for different sweep speeds, are taken from potentiometers connected across one 
side of the push-pull square -wave output. 

Feedback Alignment 

In position 8 of the sweep- speed switch, the sweep generator is triggered by 
the output from the fourth counter and the recurrence rate is therefore 400 sweeps 
per second. The vertical deflection plates are connected to the grid of the third 
counter, whose step- voltage pattern (2000 sequences per second) is therefore seen 
on the display. The feedback trimmers, some of which are critical and may require 
readjustment at varying operating temperatures, are conveniently adjusted with this 
display. 

The timing circuits used with the Loran indicator are complex but ingenious, 
and may be said to represent a high degree of attainment in the art. 


12.26 


LORAN 


Summary of Displays and Sweep Speeds 


Sweep Speed 

Switch position 

Duration of each sweep* 
(microseconds) 

• 

Nature of Display 

1 

20,000 

two traces, A and B pedestals, 

A and B received pulses. 

2 

750 

Two traces (tops of pedestals). 

A and B pulses 

3 

250 

two traces. A and B pulses. 

4 

250 

traces superimposed. Adjust 
pulses to coincide. 

5 

250 

two traces, video pulses replac- 
ed by 10- microsecond and 50- 
microsecond markers. 

6 

750 

two traces, 10, 50 and 500- micro- 
second markers. 

7 

20,000 

two traces, pedestals, 50 micro- 
second, 500 microsecond, and 

2500 microsecond markers. 

8 

2,500 

Third counter step waveform 




includes fly-back time. 



LORAN 


12.27 


Bibliography 


Identification 

Classification 

Title / 

Issued by 

JEIA 7292 

Secret 

Report of Loran Tests on 
8th Air Force Heavy 
Weather Reconnaissance 
Missions 

BBRL 

ASE no. 223 

Secret 

Trials of Loran Inter- 
ference with Port Wave 
W/T and R/T Communi- 
cation 

ASE 

IRPL-R7 

Confidential 

Second Report on Experi- 
mental studies of Iono- 
spheric Propagation as ap- 
plied to the Loran System 

IRPL 

JEIA 6914-6915 

Secret 

Effect of Loran Inter- 
ference on range of 
radio signals 

British Post 
Office 

Loran Memo 116 

Secret 

Flight tests over 

Bermuda 

MIT 

Loran Memo 122 

Secret 

Report on three- line 
fixes 

MIT 

Loran Memo 28 

Confidential 

Service areas of Loran 
pairs and chains 

MIT 

Loran Memo 134 

Confidential 

Notes on 2 me Loran 
Propagation 

MIT 

Loran Memo 1 37 

Confidential 

Optimum bandwidth for 
Loran receivers 

MIT 

Loran Memo 26 

Confidential 

Determination of errors 
in the Loran System 

MIT 

Loran Memo 138 

Confidential 

Index of Loran Reports 
and Instruction Manuals 

MIT 

AWAS no. 17 

Secret 

Notes on the Loran Sky- 
wave delay 

AWAS 

Dwg. no. A- 269 3 

Secret 

Block diagram, Indica- 
tor Timer and Receiver 
LRN no. 1 

MIT 

Loran Report 
no. 499 

Confidential 

Elements of Loran 

MIT 

JEIA- 8417 

Secret 

Aids to Navigation Memo- 
randum no. 7 

Coastal 

Command 


12.28 


LORAN 


Bibliography (cont.) 


Identification 

Classification 

Title 

WA 993 2a(4) 

Secret 

Ionospheric notes on SS 
Loran proposals 

NAR; X5016 

Secret 

Considerations of the appli- 
cation of the SS Loran 
scheme in ETO by RAF 

BDU/S. 566/R 

Secret 

Trials of SS Loran Chain 

Loran Report SS 1 

Secret 

Proposal for a Loran 
System using sky-wave 
synchronization 

11-6/22/43 

Secret 

Notes on the SS Loran 
System 

11-8/2/43 

Secret 

Flight of the Texan, July 20- 
26, 1943 

JEIA 8816 

Secret 

European and Eastern 
Atlantic SS Loran Chain 

Loga Q-392 

Secret 

Proposed Antenna Design 
for LF Loran 

WA-4052 5 

Secret 

Proposals for European 
and Eastern Atlantic SS 
Loran systems 

p:iA 10,001 

Confidential 

Suggested Improvements 
for Loran system 

JEIA 10,036 

Secret 

Loran trials in Mosquito 

VI and Oxford aircraft 

NAVAER 

00- 80V- 48 

Restricted 

Pocket handbook of air- 
borne Loran 

JEI4 9435 


Report of Loran opera- 
tional tests aboard USS 
Plunkett 

AN 16-30 APN4-3 

Restricted 

Handbook of Maintenance 
Instructions for Radio 
set AN/APN 4 

AN 08-30 APN 4-2 

Restricted 

Handbook of Operating 
Instructions for Radio 
set AN/APN 4 

HO Misc 11,701 

Confidential 

Report of Atlantic Loran 
Research Flight 


Issued by 
Air Ministry 

NAR 

BDU, RAF 
MIT 

MIT 

■ MIT 

NAA London 

Chief Signal 
Officer 

BBRL 

NRL 

Naval In- 
telligence 

CNO, U.S. 
Navy 

BUSHIPS 

U.S. War 
Dept. 

U.S. War 
Dept. 

H.O. 


LORAN 


12.29 


Bibliography (cont.) 


/ 

V 


Identification 

Classification 

Title 

Issued by 

S 67-5 Serial 
001695/P20 

Secret 

Comparison of various 
navigational systems 

(Letter: 
J.A .Pierce 
to L.A.Du- 
bridge) 

Report 625 

Secret 

The Future of Hyperbolic 
Navigation 

MIT 






Decca Navigational System 


13.01 


Type of system / 

Differential distance or hyperbolic system. 1 

Useful range ' 

Day - 1 500 miles (estimated) 

Night - 1500 miles (estimated). 

Accuracy 

Theoretical accuracy: 

Lateral error at 400 miles + .027 miles 
at 1000 miles + .068 miles 
Practical accuracy: 

Lateral error at 400 miles + .05 miles 

at 1000 miles - no data available 

Ambiguities : Complete ambiguity between closely spaced lines. Must have good 
DF fix or know point of departure. If meters are set at known point of departure 
and continue to operate there is no ambiguity. A system of sector identification 
has been worked out. No details of this system are available. 

Frequency 

20 kcps to 200 kcps. 

Wavelength 

1500 meters to 15,000 meters. 

Bandwidth 

Ground station: Single frequency. 

Receiver: Single frequency. Three frequencies, two of them related to the 
third by simple fractions such as 3/2 and 4/3 are required. 

Presentation 

Line of position (from one pair of stations, master station and one slave) 
indicated on dial* type phase meter (similar to a watthour or gas meter). Second 
line of position (from master and second slave station) indicated on second phase 
meter. Meters give continuous indication and no adjustments are necessary to take 
readings. Both lines of position are available simultaneously. 

Skill 

Ground: Well-trained operators to maintain phase lock at slave stations. 
Craft: Little skill required. Intelligent use must be made of indications so 
ambiguities can be resolved and blind faith in indications will not obtain. 

Equipment required 

Ground: 2 C.W. transmitters for a line of position and 3 C.W. transmitters 
for a fix. Master station relatively simple. Slave stations are rather complicated 
and specialized. Low frequencies used require large and expensive antenna system. 

C raft: Ver y specialized equipment including two radio-frequency amplifiers 
for line of position or three radio-^frequency amplifiers for fix. Two or four fre- 
quency multipliers and one or two integrating phase meters. 

Weight: 85 pounds - portable model is being produced to weigh 25 pounds. 

Present status 

Experimental. 

Description of system 

Differential distance may be measured by measuring the difference in the 


13.02 


Decca Navigational System 



Fig. 13-01 Fundamentals of system 


Decca Navigational System 


13.03 


timeof arrival of pulses from a master and a slave station. It may also be measur- 
ed by comparing the phase of radio-frequency signals from a master and a slave 
station. This latter phase comparison method is used in the Decca system. 

A simple explanation based on Figure 13-01 may be used even though in 
practice some complications must be introduced to make the system workable. 

A is the master station and B is the slave station. For a simple explanation 
we can assume that they both radiate a signal of 340 kcps (wavelength of 882.3 me- 
ters) and that these two radiations are exactly in phase. At the point C, bisecting 
the line between the stations A and B,the signals from the two stations travel an 
equal distance and are therefore in phase when they arrive. This same condition 
applies to any point on the line DCE . Let us now consider the point F . If this is assumed 
to be 441.15 meters (one half wavelength) to the right of point C,the distance AF 
will be one wavelength greater than the distance BF. The signals from A and B are 
therefore in phase at F. The curve GFH is such that any point Y on it will be one 
wavelength closer to B than to A. The areas between lines of zero relative phase 
angle are called "lanes". If one moved from C to F along the line AB the relative 
phase angle would go from 0® to 360^ going through 180^ at the point J. In moving 
from X to Y the relative phase angle also goes from 0° to 360®. In this system an 
integrating phase meter is used. If it were set to zero at X and then moved to Z 
along any path it would read 720^ phase shift. This integrating phase meter has no 
spring return to zero and will therefore maintain its reading if the signal is inter- 
rupted for any reason. This makes it possible for this system to function on very 
poor signals. The signals may disappear completely for short periods but when 
they reappear they turn the phase meter to the correct reading. If the phase meter 
has been reading zero on the signals at point C and these signals are absent as the 
craft moves from C to a point slightly to the right of J the phase meter will indicate 
0® instead of 360® when the point F is reached. In general if the signals are absent 
during the time that the craft moves slightly more than one half a "lane" the indica- 
tion will be in error by one "lane". 

Since it is impractical, and in fact almost impossible, to receive simultan- 
eously, but separately, two signals of the same frequency from two stations, the 
method used in the above simplified explanation cannot be used. Instead two dif- 
ferent frequencies that are simply related to the 340 kcps may be transmitted from 
two stations and received separately and simultaneously. The two frequencies used 
could be 340/4 kcps = 85 kcps and 340/3 kcps = 113 1/3 kcps. (See Fig. 13-02). At 
the craft the 85-kcps signal frequency may be multiplied by 4 to yield 340 kcps and 
the 113 1/3 kcps yields 340 kcps when multiplied by 3. This method is exactly 
equivalent in phase measurements to the simplified explanation above. 

In order to obtain a fix two sets of lines of position are necessary so that 
another phase comparison system is necessary. The frequency at which this com- 
parison is made may be 255 kcps. Since this is 3 x 85 kcps the master signal fre- 
quency of 85 kcps can be used to provide one of the 255- kcps voltages. The other 
can be provided from a third station transmitting a 127.5-kcps signal. This can be 
multiplied by 2 to yield a 255-kcps voltage. 

Three fixed transmitters and their associated control circuits are required 
on the ground. The equipment on the craft comprises three phase-stable amplifiers, 
four frequency- multipliers and two integrating phase- meters. 

The fixed ground equipment consists of a master transmitter and two slave 
transmitters. The master transmitter A is crystal-controlled and special provision 


13.04 


Decca Navigational System 



Fig. 13-02 Diagram of triplet 


is made for keeping the phase of the radiated signal constant with respect to the 
crystal. The frequency of this master station A is 85 kcps. At slave station B 
this 85»kcps signal from the master station is received and amplified and its fre- 
quency is multiplied by 4/3 and the resulting frequency of 113 1/3 kcps is used to 
drive the transmitter. A phase-locking system is used to compensate for random 
phase variations in the transmitter and antenna. 

At slave station C the 85-kcps signal is received and multiplied by 3/ 2 and 
the resulting frequency of 127.5 kcps is transmitted. A similar phase-lock system 
is used here. 


Decca Navigational System 


13.05 



DECOMETER NO. 2 


DECOMETER NO. I 


Fig. 13-03 Block diagram of receiver- indicator 


Figure 13-03 is a block diagram of the receiver indicator. The accuracy 
of measurement depends upon the amount and constancy of the phase shift in the 
various elements of the system. In order to provide a means for checking and ad- 
justing the phase shift of the three channels a phase-reference oscillator is provid- 
ed. In the example given the frequency of this oscillator is 14.166 kcps. The 6th 
harmonic is 85 kcps, the 8th harmonic is 113 1/3 kcps, and the 9th harmonic is 
127.5 kcps. Since the 14.166-kcps output consists of very sharp pulses, all these 
harmonics are inherently in phase when multiplied to a common comparison fre- 
quency so that the two decometers should indicate zero phase shift. A switch is 
provided to enable this check to be made whenever desired. The exact circuit of 
the phase discriminator and decometers is not available but a possible circuit is 
that of Figure 13-04. Two phase- discriminating rectifier circuits are used. The 
output of each phase- discriminating rectifier controls two DC amplifier tubes. 
The plate currents of these two tubes flow through two differentially- wound coils in 
such a way that the field is zero if the plate currents are equal. If the phase- dis- 
criminating rectifier supplies control voltages to the DC amplifier tubes the plate 
current becomes unbalanced and a net field is established in the coils. The magni- 
tude and sense of this field depends upon the relative phase of the two applied sig- 
nals and is in fact proportional to the sine of the phase difference. The second 
phase- discriminating rectifier and DC amplifiers control the field of a second set 


13.06 


Decca Navigational System 


of coils located at right angles to the first set of coils. The magnitude and sense 
of the field in this second set of coils depend upon the relative phase of the signals 
applied to it. The 340 kcps from the A station is applied to both phase- discrimina- 
tors in phase. The 340 kcps from the B station is applied to the two phase-dis- 
criminators 90° out of phase thus producing a flux component proportional to the 
cosine of the phase difference. These two crossed sets of coils set up a field whose 
direction indicates the phase angle between the signals from the A and B channels. 
A small permanent magnet is pivoted in this field and is geared to indicating point- 
ers. This magnet indicates the direction of the field and therefore the relative 
phase of the A and B signals. The geared indicators integrate the phase shift. 

Figure 13-05 is the block diagram of a typical slave station giving details 
of the phase- lock system. An antenna or loop (a) picks up the 85-kcps transmission 
from the master station. This is so placed and orientated that it has a maximum 
response to the master station' s signal and a minimum to its own transmitting an- 
tenna. This signal is amplified by a phase- stable amplifier and is then multiplied 
by 4/3 to yield the 113 1/3-kcps signal used to drive the transmitter. This can be 
accomplished by dividing the 85-kcps frequency by 3 and then multiplying this re- 
sultant 28 1/3 kcps by 4. This 113 1/3-kcps signal is then fed through an electronic 



Fig. 13-04 Suggested phase meter circuit 





Decca Navigational System 


13.07 


phase-shifter and then to the transmitter. In order to maintain thej correct phase 
of transmission an automatic phase- locking monitor is used. Two phase- stable 
amplifiers are used. One amplifies the 85-kcps signal from the receiving antenna 
A. The other amplifies the 113 1/3-kcps signal from a loop near the transmitting 
antenna. The 85-kcps frequency is then multiplied by 4 and the 113 1/3-kcps fre- 
quency is multiplied by 3 so that two 340'kcps frequencies are produced for phase 
comparison. These two voltages are applied to a phase- discriminator similar to 
that used in the receiver- indicator. A DC control voltage obtained from the phase 
discriminator is used to control an electronic phase- shifter in the transmitter 
channel. A decometer is also connected to this phase discriminator. The electronic 
phase shift works in such a way that it tends to keep the decometer reading zero. 
The phase of the 340 kcps derived from the 113 1/ Sleeps voltage relative to the 
340 kcps derived from the 8&-kcps voltage therefore depends upon the relative 
phase shifts in the two channels of the phase- locking monitor. Since zero relative 
phase between the 340 kcps derived from the 85 kcps and the 340 kcps derived from 
the 113 1/ 3-kcps transmission may not be that desired, it is possible to establish what- 
ever phase is desired by a manual phase control in the 113 1/3-kcps channel. This can 
be set and checked by switching the inputs of the two channels to the phase-refer- 
ence oscillator and adjusting the manual phase control for the proper decometer 
reading. The electronic phase shifter is disconnected while this check is made 
so that the transmitter phase will not be greatly disturbed. A manual phase cor- 
rection control is provided in the transmitter channel to correct long term phase 
shifts. Thus the electronic phase control only has to correct the phase shift due 
to antenna sway, voltage variations, and so forth. 



Fig. 13-05 Block diagram of slave station 









13.08 


Decca Navigational System 


Identification 
JEIA 7080 

JEIA 7081 


Classification Title 

Secret Investigation of Sonne and Decca 


Secret Further Notes on Decca Naviga- 

tional System 


Issued by 

Intelligence 

Division 

C.N.O. 

Intelligence 

Division 

C.N.O. 


POPI 


14.01 


POPI (Post-Office Position Indicator) 


Type of system 

Differential Phase (hyperbolic position lines). 

Useful Range 

Depends on siting and power of transmitter and on height of receiver. A range 
of 1500 miles over sea by day and by night is considered easily attainable. 

Accuracy and Precision 

The theoretical precision attainable depends on the antenna spacing used and 
on the distance of the craft from the transmitter. See Table 14-01, page 14.07. The 
accuracy attainable in practice under full-scale operating conditions is not predict- 
able, due to the tentative nature of present development. 

Presentation 

Several types of presentation have been proposed. One uses an adjustable 
phase-shifter and a meter. The operator adjusts the phase-shifter for a null on the 
meter, andthe line of position is then read from graduations on the scale of the phase- 
shifter. Another uses two pointer-and- scale meters, which together give indication 
of a line of position. 

Operating Skills Required 

(a) If the direct- reading meter type of presentation is used, the only operations 
required at the craft are the tuning of a radio receiver to the frequencies of the bea- 
con transmitters used, (b) For ground- stations, monitoring is required. 

Equipment Required 

(a) Ground: Each beacon consists of four antennas driven by two transmitters , 
spaced as discussed below. The transmitting equipment could be transportable. Two 
beacons are required for a fix. (b) Craft: A normal communications receiver is 
required. Automatic volume control and an IF crystal filter are desirable but not 
essential. In addition, a special POPI indicator is required. This can be of the 
direct- reading type (pointer and scale) and in its simplest form does not require any 
additional tubes apart from those in the communications receiver. The indicator is 
suitable for use by the pilot of an aircraft and is easily adaptable for homing and for 
blind landing, (c) Monitoring: Each beacon requires a monitor station, located near 
the beacon. Control of the beacon transmission from the monitoring point could be 
made fully automatic but this has not so far been attempted due to the limited scale 
of the trials made. 

Radio- Frequency Spectrum Allotments Required 

This system has been tested on a small scale at a frequency of about 750 kcps 
(400 meters). The frequency used is not critical as far as the system is concerned, 
and the choice would presumably be governed by the coverage required. Since the 
transmissions are CW, with slow keying, the bandwidth required is of the order of 
1 kcps. 

Present Status 

This system has been tried out experimentally, on a small scale, the receiv- 
ing equipment being in a road vehicle. So far as we are aware, no full scale tests 
have been carried out, nor has the equipment been air- or water-borne. 

Principle of Operation 

The four antennas of a beacon are arranged at the corners and center of an 
equilateral triangle (see Figure 14-01). Antennas A, B and C are fed from a central 


14.02 


POPI 



Fig. 14-01 POPI beacon 


transmitter through suitable keying, phase-shifting and power-amplifying stages. 
Figure 14-02 shows a block diagram of a suitable arrangement. 

Referring to Figures 14-01 and 14-02, the A, B and C antennas radiate an un- 
modulated signal at the frequency f^. These three transmissions are keyed at a 
slow rate so that the sequence is as follows: transmission from A, transmission 
from B, transmission from C, space. See Figure 14-03. 

The rate of keying used is such that five complete cycles of the sequence 
occur per second. Each individual transmission, and the space where no signal is 
transmitted, would then be of 1/20 second duration. The keying is accomplished elec- 
tronically, using a second oscillator (of low audio-frequency f 2 ) followed by a fre- 
quency divider (dividing by n) and pulse generator. The output of this same audio- 
frequency oscillator is mixed with the output of the RF oscillator and selectively 
amplified. The resulting signal, which is unmodulated but of frequency f^ +f2 is radi- 
ated continuously by the fourth antenna D. 

The phases of the signals transmitted by antennas A, B,and C may have any 
desired relationship, but it is assumed for purposes of explanation that the phases 
are identical. Referring to Figure 14-01, a receiver situated at P, on the perpendi- 


POPI 


14.03 



A 


B 


C 


D 


Fig. 14-02 Block diagram — POPI beacon 


cular bisector of the line joining B and C, will receive the B and C signal in phase 
since the distances BP and CP are equal. The same will be true of points P' and 
P" if the transmissions from A and B and from A and C respectively are consider*- 
ed. At Q there will be a phase difference between the received B and C signals. 
The locus of all points for which this received phase difference is constant is the 
hyperbola qq. There is therefore a family of hyperbolae of constant phase difference 
for the B and C signals. This follows from exactly the same fundamental reasoning 
as that which applies to the Loran, Gee and other "hyperbolic” systems, since time 
difference in a pulse system and phase difference in a C ,W system are fundamen- 
tally the same. Thus a craft equipped with a receiver and POPI indicator giving 
the relative phase of the B and C signals can locate itself on one of these 
hyperbolic position lines. Similar families of hyperbolae exist for the A and B 
transmissions and for the A and C transmissions - three families of hyperbolae in 
all. Sector ambiguity with regard to the B and C transmissions is solved by a read- 
ing taken on either the A and B or the A and C positions. A unique position line with 
respect to the site of the beacon is thus obtained. 



ABC space ABC space ABC 


time 


Fig. 14=03 Transmission sequence 






14.04 


POPI 


With the antenna spacing proposed, the three families of hjrperbolae associated 
with a particular beacon degenerate into radial straight lines (with negligible error) 
at distances which are small compared with the maximum working range proposed. 
For this reason, the loci of constant phase difference shown in Figures 14-04 and 
14- 05 are drawn as radial straight lines. It should be realized that these are actually 
hyperbolic, and due attention should be paid to this fact in the layout of charts to be 
used with this system in areas close to the transmitters. 

If readings on a second similar beacon are taken, the intersection of the two 
position lines gives a fix. 

The problem at the receiver is therefore to compare the phase of two trans- 
missions occurring on the same radio frequency but consecutively in time. The 
radiation from the fourth antenna (D) is a continuous unmodulated carrier wave of a 
slightly higher frequency. The audio output of the receiver will therefore be the 
difference frequency, or f 2 the original audio frequency. The phase of the carrier of 
frequency fj will be preserved in the phase of the audio output f2. The problem at 
the receiver therefore resolves itself into phase comparison of two audio-frequency 
signals representing the beat signals from B and D and from C and D respectively. 

The fact that differences in RF phase are preserved as differences in audio 
(beat note) phase may be proved as follows: 

Let the received signals be given by: 

E| sin coi t from B 

E2 sin (W| t + <>) from C 

Eg sin [ (ct)^ + (1)2)1 + 0] from D 


Assume also square- law detection. Then during the B transmission the out- 
put from the detector will contain the signal 

|Ej sin 0)^ t + E3 sin [ (cj^ + | ^ 

The audio-frequency term of this expression is given by 

Ej E3 cos (0)2 t + 0) (1) 


During the C transmission the output from the detector will contain the signal 


|E2 sin ((*)j t-H^) +Eg sin [(0)^ +0)2)! + 0]|' 


And the audio-frequency term is given by 


E2 E3 cos (ci)2 t + 0 -</)) (2) 

It will be seen that the phase difference between (1) and (2) is<t> , and this is 
the same phase difference as thkt between the original B and C transmissions. 

Spacing of position- lines obtained 

The configuration of the hyperbolic position lines (degenerating into radial 
lines at a distance) depends on the antenna spacing. Four points are of interest in 
this connection: 

(1) If the spacing between antennas is greater than one-half wavelength, there is a 
sector ambiguity. Consider for example the case where the spacing is two wave- 
lengths. The loci of constant phase difference are as shown in Figure 14-04, where 


POPI 


14.05 


PQ represents the line joining one pair of antennas and the radial lines are loci of 
constant phase difference. The choice of + or - sign at any particular point depends 
on whether the phase of B is measured relative to that of C or vice versa. There is 
ambiguity between the right-hand and left-hand halves of the diagram, but this is 
present in all hyperbolic systems and it is assumed that a navigator will know whether 
he is east or west of the beacon location. However, in addition to this the following 
ambiguities exist in Figure 14-04: 

(a) It will be seen that any particular reading occurs twice in the right-hand 
half of the diagram. For example, -240^ occurs in both the sectors PR and 
RS. (It is assumed that the indicator used will be able to distinguish between 
+240^ and -240®, i.e. whether B is leading C or C leading B. This is taken 
care of in the indicator to be described in connection with POPI). 

(b) There is also ambiguity as between -240® and +120^ since the navigator 
knows only the existing phase relationship and not the process by which it got 
that way. Thus there is a four-fold ambiguity in Figure 14-04. 

If the spacing is between one wavelength and one half of a wavelength, there 
is still a two-fold ambiguity. If the spacing is reduced to one half of a wavelength or 

P 


-360 ORO 



14.06 


POPI 


P 


-180 



Q 


less, there is no ambiguity. Figure 14-05 shows the phase loci for a half -wavelength 
spacing. 

(2) The accuracy of position discrimination for a given minimum phase discrimina- 
tion is not uniform. It is greatest along the line OS which is the perpendicular bisec- 
tor of the line joining the transmitting antennas, and least (zero) along the line PQ 
joining the antennas. This is true for all antenna spacings. However, within an arc 
60° on either side of OS the attainable precision does not depart too far from the 
maximum value (one half). Exact figures on theoretical precision are given in Table 
14-01. 

(3) The maximum accuracy attainable (along the line OS) is greater with wide anten- 
na spacing than with narrow spacing. Table 14-01 gives calculated results for various 
spacings, both in the direction of maximum accuracy (OS) and also in a direction 60° 
from this. It should be emphasized that these figures represent theoretical accuracy 
only. In the interests of removing ambiguity, the system to be described further 
assumes an antenna spacing of 0.5 wavelengths (line 1 in Table 14-01). 

(4) At distances greater than about five times the antenna spacing, the hyperbolic 
position lines are so nearly straight that negligible error is introduced by making 


POPI 


14.07 


Antenna 

Spacing 

(Wavelengths) 

Max. discrimination (on normal) 
for 2® phase discrimination 

Discrimination (60^ off normal) 
for 2^ phase discrimination 

degrees azimuth 

Miles at 
1000 miles 

degrees azimuth 

Miles at 
1000 miles 

0.5 

0.636 

11.2 

1.272 

22.4 

1.0 

0.318 

5.6 

0.636 

11.2 

2.0 

0.159 

2.8 

0.318 

5.6 

4.0 

0.079 

1.4 

0.159 

2.8 

10.0 

0.032 

0.56 

0.064 

1.12 


Table 14-01 

this assumption. Figure 14-04 was drawn under this assumption, and also Figure 
14-05, which shows lines of equal phase difference for a spacing of 0.5 wavelength be- 
tween antennas. The absence of ambiguity and the reduction in maximum accuracy 
of discrimination will be noted. 

Craft Equipment: The receiver is tuned to the frequency of the carrier. Since the 
frequency difference between the D transmission and that from A, B, or C is small, 
and the keying rate slow, the bandwidth required is small and a crystal filter might 
be used if the signal- to- noise ratio is poor. In the following discussion, the audio 
frequency is assumed to be 80 cps and the switching rate 5 sequences per second. 

After detection, the audio signal will consist of three consecutive dashes of 
80 cps tone followed by a blank space. The relative phase of the three carriers re- 
ceived will be preserved in the relative phases of the 80 cps dashes, as previously 
proved. The problem is now to compare the phase of one 80 cps dash with that of 
another 80 cps dash which occurs at a different time. To do this accurately does not 
appear to be easy and in our opinion this stage in the operation of the system pre- 
sents the greatest difficulty in regard to reliability, accuracy and simplicity. An 
outline of the proposed scheme follows. 

Referring to Figure 14-06, the audio output from the receiver is applied to a 
rotating switch with four contact sectors. The rotating arm is driven (through reduc- 
tion gearing) from a synchronous motor which is in turn driven by an 80-cps oscilla- 
tor. This oscillator is synchronized through a phase- shifting circuit from one of 
the 80-cps outputs from the rotating switch. The desired condition is that the four 
contacts in the switch shall receive respectively the A, B and C signals and the no- 
signal space; i.e. the periods of time during which the four sectors are successively 
in contact with the rotating arm shall be synchronized with the four periods in each 
received cycle of events. This condition is indicated by zero deflection of the meter 
M which is a direct- current meter fed with the smoothed, full- wave rectified output 
from the fourth sector. This indication is obtained by changing the adjustment of 
the phase shifter and therefore the phase of the synchronous motor. The separated 
A, B and C outputs are filtered and may then be amplified as indicated. 

However, the authors of the original scheme were anxious to preserve maxi- 
mum simplicity' in the additional indicating equipment required. For this reason 


14.08 


POPI 


FROM 



A OUTPUT 


B OUTPUT 


C OUTPUT 


M 


f A ^ 

1 ATOD 


VARIABLE - PHASE 


UoUILL A ! UK 

SYNCH 

SHIFTER 


SYNCHRONOUS 

MOTOR 


Fig. 14-06 Block diagram- -received signal separation 


they designed and used a type of phase-comparison indicator which requires no extra 
tubes and no additional power supplies. Using this instrument the additional ampli- 
fiers shown in Figure 14-06 would not be used. This type of indicator is illustrated 
in Figure 14-07, and operated as follows: 



Fig. 14-07 



POPI 


14.09 


For phase comparison, one of the three outputs is selected and passed to a 
band-pass filter tuned to 80 cps and having a bandwidth of 2 cps. This in effect con- 
stitutes a ringing circuit and it is desired that the 80-cps output from the circuit shall 
continue during the absence of an input signal. The phase of this output is dictated 
by the samples of 80-cps signal periodically fed to the input. Another of the switch 
outputs is applied to a calibrated phase-shifter (not to be confused with the phase- 
shifter of Figure 14-06). The output from the phase-shifter and that from the ringing 
circuit are then compared by a phasemeter and the phase-shifter adjusted for zero 
phase difference. The difference in phase between the two channels selected is then 
read from the calibrated phase-shifter and a line of position thus selected, using 
charts on which the lines of POPI equal phase-difference have been overprinted. 

Referring to Figure 14-07, the calibrated phase shifter consists of a synchro 
unit, whose three fixed windings are suitably fed, and from the rotor of which a vari- 
able-phase signal is taken. This signal and the output from the band pass filter (or 
ringing circuit) are supplied to the phase- sensitive rectifier shown. The meter M* 
will read zero if the two inputs are 90® or 270^ out of phase. These circuits are 



Fig. 14-08 Phase differences ^t receiver 


14.10 


POPI 


taken from the proposals in the original paper in which the principal advantage 
claimedwas the absence of tubes and power supplies other than those in the receiver 
itself. The circuits do not necessarily represent the most efficient and accurate way 
of accomplishing the phase comparison; and it is our opinion that, should this system 
be developed further, other phasemeters could be tried with advantage, for example 
that described in the report on the Decca system. It is also desirable to use a direct- 
reading phasemeter which does not involve matching for a null reading on a meter. 

Sector Identification 

Assume now that this difficulty has been overcome and that a suitable direct- 
reading phasemeter is available. The following discussion summarizes the original 
proposals. Referring to Figures 14-06 and 14-07, it will be seen that there are three 
outputs from the rotating switch and two inputs to the phase-comparing circuits. The 
layout of the beacon antennas is such that there are three sets of hyperbolic position 
lines (which degenerate into great- circle position lines at some distance out from the 
beacon), one set for each pair of antennas. There is thus an opportunity to make use 
of only the sector of maximum discrimination in each case. 

Figure 14-08 illustrates the phase differences between pairs of received sig- 
nals at 30® (azimuthal) intervals. The antenna spacing for each pair is assumed to be 
X/2. The numbers in the outer ring are arbitrarily numbered sectors. The next ring 
(AB) gives the phase of B relative to A, the next ring (BC) the phase of C relative to 
B and the inmost ring (CA) the phase of A relative to C. The phase of A relative to 
B will of course be the reverse of AB and will be denoted by BA, the first letter in 
each case giving the phase reference. An observer in sectors 1 should use the A and 
B signals for maximum precision. Likewise an observer in sectors 2 should use the 
A and C signals, and so forth, as indicated by the thick lines enclosing the sectors. 
In either case the observed phase angle for the chosen pair lies between -90® and 
490®, i.e. in the first or fourth quadrants of a direct- reading phasemeter with 360® 
scale. 


Table 14-02 lists the quadrants in which the phase angles observed will lie 
for any pair of signals. 


Sectors 

AB 

BA 

BC 

CB 

CA 

AC 

1 

4 or 1 

1 or 4 

3 

2 

2 

3 

2 

2 

3 

3 

2 

1 or 4 

4 or 1 

3 

2 

3 

4 or 1 

1 or 4 

3 

2 

4 

1 or 4 

4 or 1 

2 

3 

3 

2 

5 

3 

2 

2 

3 

4 or 1 

1 or 4 

6 

3 

2 

1 or 4 

4 or 1 

2 

3 


Table 14-02 



POPI 


14.11 


A six-position selector switch may be used which will allow any of the combinations 
(AB, BA, BC, etc.) shown to be selected. If the phasemeter used is graduated in 
quadrants 1 and 4 only, then a rotation of the switch until a reading is obtained ensures 
that the correct pair of signals will be selected, but ambiguity is now present as be- 
tween AB and BA, BC or CB, and CA or AC. It is necessary to have all six combina- 
tions available on account of the direction of rotation of the meter. Consider a craft 
navigating a circular course startingfrompointP in Figure 14-08 and proceeding clock- 
wise about the beacon as center. In sectors 1, the phasemeter reading changes from 
-90^ to +90 if the phase of B relative to A is measured (AB). In sectors 4 however, 
if the same phase difference (AB) is measured, the meter reading will change in the 
opposite sense, i.e. from + 90 ° to -90°. To avoid the necessity of having two phase 
scales reading in opposite directions, it is therefore necessary to have all six com- 
binations available. 

The ambiguity between two of the six switch positions may be solved by having 
a second or subsidiary phasemeter which indicates a reading only in the third (or 
second) quadrant. This involves the use of a second six-position switch ganged with 


Switch 

Position 

Main Meter 

Subsidiary 

Meter 

1 

AB 

AC 

2 

AC 

BC 

3 

BC 

BA 

4 

BA 

CA 

5 

CA 

CB 

6 

CB 

AB 


Table 14-03 


Switch 

Position 

Main Meter 

Subsidiary 

Meter 

1 

3 

4 

2 

4 

2 

3 

2 

2 

4 

2 

1 

5 

i 

3 

6 

3 

3 


Table 14-04 

Meter indications for various switch positions, observer 
in sector 5b (point Q, Figure 14-08). 




14.12 


PQPI 


the first. The connections obtainable in the six positions of the switch, for both mam 
and subsidiary phasemeters, are indicated in Table 14-03. Consider now an observer 
situated at (say) Q in sector 5b. The quadrants of the phase differences to be indicated 
by the two phase meters for each of the six switch positions are shown in Table 14-04. 

If the main meter is graduated in only the first and fourth quadrants, and the 
subsidiary meter in only the third quadrant, it will be seen that only position 5 on the 
selector switch will give a readable indication, and the ambiguity is resolved. 

The physical connections from the selector switch to the phasemeters will 
depend on the type of direct- reading phasemeter used. The procedure to be followed 
in obtaining a fix would then be as follows: 

1. Tune receiver to selected beacon frequency. 

2. Adjust output level if necessary. 

3. Adjust phasing of rotary switch until meter M (Figure 14-06) reads zero. 

4 . Rotate six-position selector switch until a reading is obtained on both phasemeters . 

5. Read main phasemeter, and note time. 

6. The switch position gives the sector number (1 to 6, Figure 14-08) and the main 
phasemeter reading gives (by reference to a conversion table or chart based on 
Figure 14-05) the azimuth angle within the sector, yielding a position line. 

7. Repeat the above procedure using another beacon. The intersection of the two 
position lines gives a fix. 

It should be possible to make a good deal of the above procedure automatic 
if suitable control circuits are used. 

Since the two lines of position are not obtained simultaneously, running fix 
technique will be necessary. 


Bibliography 




Identification 

Classification 

Title 

Issued by 

WA 781-6 

Secret 

Post-Office Engineering Department 
Radio Report No. 928 

British Post- 
Office, London 

WA 781-2 

Secret 

” No. 929 

II 

WA 1804-6 

Secret 

'' No. 1077 

II 

WA 2523-5 

Secret 

" No. 1085 

II 


A-N Radio "Range 


15.01 


The A-N type of radio range has been very widely used in this country by 
civil aviation. 

Each ground station sets up four tracks or ranges. This is accomplished 
by using a directional antenna system that can emit two different patterns. In 
Figure 15-01 these two patterns are indicated as the solid- line pattern and the 
dashed-line pattern. The transmitter power is keyed alternately from one 
pattern to the other. The keying is such as to produce A' s (.-) from the solid- line 
pattern and N's (-.) from the dashed-line pattern. This keying is interlaced in 
such a way that an aircraft on the line OC would receive equal signals from the A 
and N patterns and a continuous tone would be heard. An aircraft on the line OE 
would receive an A signal proportional to the length OF and an N signal pro- 
portional to the length OG. Thus the A signals would predominate and the pilot 
would know that he was off course to the right if he is flying toward O. 

The patterns of Figure 15-01 can be obtained by using two loops at right 
angles to each other. One loop will produce the "A" pattern and the other will pro- 
duce the "N" pattern. These patterns may also be obtained by using four tower 
antennas located on the corners of a square. The diagonal of the square is small 
compared to a wavelength. Diagonally opposite towers are fed 180° out of phase 
from a common feed point. Each diagonally opposed pair of towers will give a 
field-pattern similar to a loop. This system minimizes high-angle radiation and 
will therefore considerably reduce the sky-wave errors experienced with the cross- 
ed loops. 

In Figure 1 5-01 the opposite courses are 1 80° apart and the adjacent courses 
are 90° apart. It is very seldom that four courses having this angular relationship 
are desired. The 90° angle between adjacent courses may be modified by attenuat- 
ing the energy fed to one loop or pair of diagonally opposed antenna towers 
thus yielding a pattern similar to Figure 15-02. The 180° relation between opposite 
courses may be altered by adding an omnidirectional vertical antenna to the crossed 
loop system or at the center of the square of the 4-tower system to yield a pattern 
as shown in Figure 15-03. By proper adjustment these four courses can therefore 
be made to set up 4 airways leading from a city to other cities. 

Most of the present A-N type "ranges" use the five- tower antenna system. The 
center tower is driven by a separate transmitter whose frequency is 1020 cps 
different from the transmitter driving the four corner-towers. The transmitter 
driving the central tower can be voice modulated for transmission of weather 
information. The voice- channel of the transmitter has a filter to eliminate fre- 
quencies of 1020 cps. The radio-range receiver has a 1020-cps band-pass filter 
which will discriminate against the voice modulation and give only the A-N signals. 
A 1020-cps band-stopfilter in the receiver will reject the A-N signals and permit the 
voice modulation to be heard. The pilot can thus choose either the A-N signals or 
the voice modulation. 

The radio "range" stations in the United States operate onfrequencies between 
200 kcps and 400 kcps. These stations are spaced approximately 200 miles apart. 
Every 30 seconds a code signal identifying the station is sent alternately on the 
A and N patterns. There are approximately 200 of these "range" stations in the Uni- 
ted States. 


15.02 


A-N Radio "Range 


A B 



Fig. 15-01 Radiated patterns 



Fig. 15-02 Radiated patterns with 
course- shifting 


Fig. 15-03 Radiated patterns with 
course- bending 


The multiple courses, bent courses, and night-time sky-wave errors ex- 
perienced with the LF type of A-N radio range prompted the development of 
several VHF radio "ranges". Since the ground wave from a VHF radio "range" is 
attenuated very rapidly, course bends due to diffraction of the ground wave in 
passing over different terrain are no problem. Waves of this frequency are not 
reflected by the ionosphere and therefore there are no sky-wave errors. Inter- 
ference patterns caused by reflections from hills and mountains are not as trouble- 
some at these frequencies as in the LF system since the maxima and minima will 
be only a few feet apart. They will show up merely as a modulation on the sig- 
nal. 


CAA VHF Radio "Ranges 


15.03 


As in the case of the VHF omnidirectional- beacon, horizontal polarization 
has been found to give the best results. Alford loops are used as radiating ele- 
ments. 


A four-course aural VHF "range" has been developed. Figure 15-04 gives 
the field patterns produced. These patterns are more efficient than the crossed 
figure of 8' s produced by the LF "ranges" since the maximum energy is directed 
near the useful equisignal courses. Figure 15-05 is a block diagram of the system. 
The direction of the courses is shifted by rotating the whole array. The array is 
mounted 5/4 of a wavelength above a counterpoise screen 35 feet in diameter. All 
this is mounted on top of a 30-foot steel tower. The system operates on a frequency 
between 123 and 127 mcps with a power output of 300 watts. The transmitter is 
modulated 100% by a 1020-cps audio signal. Some of these VHF "Ranges" were 
installed on the New York to Chicago airway. 

A two-course VHF radio range which gives visual indication to the pilot 
has also been developed. The antenna field patterns produced are given in Figure 
15-06. The signalfrom the solid-line pattern is modulated with a 90-cps frequency 
and the signal from the short-dashed-line pattern is modulated with a 150-cps fre- 
quency. The output of the aircraft receiver is passed through two filters which 
select the 90-cps and 150-cps audio signals respectively. These two audio signals 
can be rectified and applied to the two windings of a differentially- wound zero- 
center meter to give course indication. Quadrant- identification is possible by 
the use of the long- dashed- line pattern and the dot-dash-line pattern. A 1020-cps 
signal is keyed to these two patterns in some specified code so that the pilot can 
identify which side of the "range" station he is on. A filter in the output of the re- 
ceiver rejects the 90 cps and 150 cps signals and passes only the 1020 cps signal. 
The course indication can be used to operate an automatic pilot. It is proposed to 
install two parallel lines of these "range" stations along busy airways to provide two 
parallel courses for aircraft flying in opposite directions. 


15.04 


CAA VHF Radio "Ranges 



Fig. 15-04 Four-course VHF aural "range 


CAA VHF Radio "Range' 


15.05 



divider 


Fig. 15-05 Loop array of VHF aural "range* 


15.06 


CAA VHF Radio "Range 



CAA A-N Radio "Range" 

Sandretto, P. C.: "Principles of Aeronautical Radio Engineering", pp. 24- 
71, McGraw-Hill Book Company, Inc., New York, 1942. 

CAA VHF Radio "Ranges" 

Sandretto, P. C.: "Principles of Aeronautical Radio Engineering", pp. 72-105, 
McGraw-Hill Book Company, Inc., New York, 1942. 


Aircraft Direction-Finders and Homing Systems 


16.01 


There are numerous aircraft direction-finders and homing systems. While 
these systems could be used on other types of craft they have been used principal- 
ly on aircraft in the past. 

Figure 16-01 illustrates the principle of a homing system. A loop and non- 
directional antenna are used to give a cardioid response-pattern. The maximum 
of this cardioid can be shifted from the right side of the craft to the left by revers- 
ing the loop connections. The two response patterns obtained are shown as the 
solid and dotted patterns of Figure 16-02 (a). 

The cardioid pattern results from the addition of the signal from the loop- 
pattern Figure 16-02 (b) and the signal from the non- directional antenna pattern 
Figure 16-02 (c). However, the signal from the loop is 90° out of phase with the 
signal from the non- directional antenna. In compensation it is necessary to intro- 
duce a 90° phase-shift in one of the channels. 

In Figure 16-01 the loop signal is alternately reversed by the action of the 
motor-driven reversing-switch. The output of the receiver is alternately connect- 
ed to the two coils of a differentially- wound zero- center DC meter in synchronism 
with the reversing of the loop input. In Figure 16-02 (a) OA represents the longi- 
tudinal axis of the aircraft. If the desired station lies along the line OB the signal 
when the dotted pattern is switched on will be proportional to OC and the signal 
will be proportional to OD when the solid pattern is switched on. Thus, a larger 
signal is applied to one coil of the differential meter than to the other coil. The 
connections are such that the meter deflects to the right indicating that the desired 
homing- station lies to the right of the heading of the craft and that the craft should 
be turned to the right to home on the station. 

If the loop is rotatable the system can be used as a manual direction finder 


Non directional 
"sense" antenna 



Fig. 16-01 Principle of switched- cardioid homing system 


16.02 


Aircraft Direction-Finders and Homing Systems 



by rotating the loop until the meter reads zero. "Sense" can be determined by not- 
ing the relative motion of loop and meter pointer. If a bearing is taken with OA 
pointing to the station a slight rotation of the loop to the left will give a meter de- 
flection to the right. However, if the station is in the direction OE the pointer will 
deflect in the same direction as the rotation of the loop. 

In practice the mechanical switching- system of Figure 16-01 is replaced 
by an electronic switching- system similar to Figure 16-03. The loop-signal is 
amplified by a tuned radio-frequency amplifier. This amplified loop- signal is 
then applied to a balanced modulator which performs the switching. This switched 
loop- signal is combined with the signal from the non- directional "sense" antenna 
which has been shifted 90^ in phase and applied to the receiver. The output of the 
receiver is applied to a directional rectifier (balanced modulator). This circuit 
is also supplied with audio frequency from the same source that supplies the RF 
balanced modulator. The output of this directional rectifier operates a zero^center 
DC meter. 

Figure 16-04 is the block diagram of a typical self- orienting automatic 
direction-finder. The loop- signal is amplified by the tuned- RF loop- amplifier. 
The amplified loop- signal is applied to a balanced modulator. The output of the 
balanced modulator, which consists of only the two side-frequencies, is combined 


Aircraft Direction-Finders and Homing Systems 


16.03 


Non-directionol 
"sense" antenna 



Fig. 16-03 Block diagram of homing system 


with the signal from the non- directional antenna which has been shifted 90® in 
phase. The resulting amplitude- modulated wave is amplified and detected by the 
receiver. The audio output is filtered by a bandpass filter to remove all modula- 
tion frequencies but that produced by the balanced modulator. This is to avoid 
overload of the motor control circuit by audio modulation and noise. If either of the 
nulls of the loop are pointing to the station the audio output will be zero. When the 
loop swings through a null the phase of the audio signal reverses. The audio out- 
put and the AC from the same source that supplies the balanced modulator are 
applied to the motor-control circuit. An antihunt voltage proportional to the deriva- 
tive of the error is applied in series with the audio voltage. This antihunt voltage 
is obtained from an armature- reaction rate- generator. *This motor- control cir- 
cuit controls a reversible two-phase motor which rotates the loop. The direction 
and speed of the motor depends upon the phase and amplitude of the audio signal 
resulting from the loop switching. This motor will rotate the loop to a null. Only 
one of the nulls of the loop will yield a condition of stable equilibrium. The loop 
position is repeated to the instrument panel by a flexible shaft or synchro- mechan- 
ism. This equipment may be used for homing by maintaining zero relative bearing 
to the station. 

Figure 16-05 is the block diagram of an automatic direction finder that em- 
ploys a rotating loop or equivalent. The loop is rotated continuously by a driving 
motor. This driving motor also drives a reference-phase generator. The signal 
from the rotating loop is amplified by a tuned radio-frequency amplifier and is then 
combined in the antenna coupling- circuit with the signal from the non- directional 
antenna which has been shifted 90® in phase. The signal from the rotating loop 
will be amplitude modulated but will contain only the two side-frequencies. Instead 
of a rotating loop two crossed loops and a goniometer may be used. The rotor of 
the goniometer is driven by the driving motor. 

* See Bond, D. S.: "Radio Direction Finders", pp. 201 ff. 


16.04 


Aircraft Direction-Finders and Homing Systems 


The carrier is re- supplied from the non- directional antenna. This is 
equivalent to rotating a cardioid response pattern at driving motor speed. The sig- 
nal supplied to the receiver is an amplitude modulated signal. The phase of the 
modulation envelope depends upon the direction of the station being received. The 
audio output phase is compared with the phase from the reference-phase generator 
in some type of phase-meter or comparator.lt would also be possible to use two 
crossed loops each feeding a balanced modulator. The modulating voltage would 
be applied to these two balanced modulators 90^ out of phase. This would give in 
effect an electronic goniometer. 

Two automatic direction-finders could be used to take continuous bearings 
on two stations and this information might be used in an automatic computer to 
give a continuous fix. A system of this type is described in Section 18. 



Fig. 16-04 Block diagram of self- orienting direction-finder 




Aircraft Direction-Finders and Homing Systems 


16.05 



Aircraft Direction Finders and Homing Systems 

Bond, D. S.: "Radio Direction Finders", pp. 117-149 and pp. 171-230, 
McGraw-Hill Book Company, Inc., New York, 1944. 

Sandretto, P. C.: "Principles of Aeronautical Radio Engineering", pp. 106-142, 
McGraw-Hill Book Company, Inc., New York, 1942. 





SONNE 


17.01 


SONNE (CONSOL) 

Type of System 

Azimuth, giving radial lines of position. 

Useful Range 

Depends on transmitter power and location of craft. Figures based on a num- 
ber of observations of German transmissions are: 

inAA transmitter power 1.5 kw, transmission over water. 

Night 2000 miles ) ^ ’ 

Owing to the frequency used, there are no altitude limitations at a distance. However, 

aircraft close to the source of transmissions will be subject to errors in position 

due to geometrical considerations. (See Section 1) 

Accuracy and Precision 

The best theoretical precision is approximately 1/ 6® azimuth in a line of pos- 
ition. 


Results of a number of test observations indicate an average operational ac- 
curacy of 1.70 by day and 2.3^ at night, corresponding to errors of 15 and 20 miles 
at 1000 miles range in a line of position. Ambiguity exists between alternate sectors, 
and must be solved by an approximate knowledge of position obtained from dead reck- 
oning based on previous bearings or from D/F observations. Large errors have 
been observed at sunrise and sunset. 

Type of Presentation 

Aural. The operator counts the number Of dots and dashes during a one- 
minute cycle. 

Operating Skill Required 

A feature of this system is that the complexity of the operations and equip- 
ment on the craft have been reduced to a minimum. No skill is required beyond the 
ability to txme a communications radio receiver, plus the ability to listen, count, add, 
subtract, and use charts . 

At the ground installation, skilled monitoring is desirable. 

Two minutes are required for determination of a line of position. A fix should 
be obtainable in four to six minutes . 

Equipment Required 

At the ground station: Transmitter of about 1.5 kw, specialized phasing and 
keying gear, two or three antenna towers of height 150 - 350 feet with the necessary 
transmission lines, monitoring equipment. On the craft: Standard communications 
receiver. 

Frequency 

250 - 500 kcps (wavelength 1200 - 600 meters). 

Bandwidth Required 

Less than that of the usual communications receiver. 1 kcps per ground station. 
Present Status 

The Sonne system has been used extensively by the Germans during the last 
two years of the war. The Allied Air Forces have also found the system reliable and 


17.02 


SONNE 


useful. Sonne is considered by many qualified persons to be in certain respects 
superior to Loran as a long-distance navigational aid for general use. British use 
of German Sonne transmission is usually referred to as Consol. Prior to the Japan- 
ese surrender a modified Sonne system (AN/ FRN - 5) was under development for 
fighter navigation in the Pacific theater. 

Principles of Operation 

A Sonne station radiates a multi- lobed pattern in which, by phase- switching 
and phase -shifting of the transmissions from three spaced antennas, certain "equi- 
signal" lines relative to the station are defined. These lines move slowly in position 
over a period of one minute . By counting the number of dots and dashes heard before 
and after the passage of the equisignal during this period, the operator at the craft is 
able to locate himself with regard to the ground station, obtaining a line of position. 
Another such observation yields a second line of position on a different station, and 
thus a fix is obtained. 


The ground station uses three antennas (A, B, and C in Figure 17-01). The 
spacings between A and B, and between B and C, are equal and are usually of about 
three wavelengths. A variety of arrangements as to phasing and amplitude of antenna 
currents is possible, but that used by the Germans is illustrated in Figure 17-02 (a) 
and (b). The amplitudes of the currents in antennas A and C are equal, and are one- 
fourth (other fractions maybe used) of that in antenna B. At the start of a one- minute 
phase- shifting period, the phase relations are as indicated in Figure 17-02 (a). Tak- 
ing the A current as phase reference, the C current has a phase of 180° with respect 
to A, while Bis at +90^. At the end of the first 5/6 second of the phase -shifting cycle 
the phases ofA and Care suddenly reversed. After an additional l/6 second the phases 
of A and C are suddenly returned to their original positions (ignoring for the moment 
the relatively slow superimposed phase- sweep described below). iThis phase-keying 
sequence is repeated at one - second intervals throughout the one- minute phase- shifting 
period. At the same time, the phases of A and C are moved slowly and uniformly in 
opposite directions, so that after 10 seconds, the A and C phases would be as shown 
in Figure 17-02 (b), and after 60 seconds the A and C phases would each have changed 
by 180® and would be reversed with respect to their initial positions in Figure 17-02 
(a). At the beginning of the next phase- shifting period (after a further interval of one 
minute) the phases start again from the positions of Figure 17-02 (a). The complete 
sequence of events in a two- minute cycle is as folh 
Phase- shifting and keying in the manner described 
Silent period (no transmission) 

Steady transmission from the center antenna alone 
including an identification signal 
Silent period (no transmission) 


for 

60 seconds 

for 

1 second 

for 

56 seconds 

for 

3 seconds 


120 seconds 


The complete sequence therefore lasts for two minutes, and bearings cannot 
be taken closer in time than this interval. 

Considering the radiation pattern at the start of the 60-second phasing period, 
the current in antenna B leads that in antenna A by 90®, and that in antenna C leads 
by 180® (Figure 17-02 (a)). The radiated pattern is then similar to Figure 17-03 
which is drawn for an antenna spacing d = 3 wavelengths, amplitude of A and C cur- 
rents one quarter of amplitude of B current. If the phases of A and C were reversed 
the pattern would be as shown in Figure 17-04. It will be seen that maxima in Figure 
17-04 occur at the same azimuth angles as minima in Figure 17-03 and vice versa. 
The pattern is symmetrical with regard to the line of antennas, but not with regard 
to a line perpendicular to this. If now the phase reversal of the A and C currents 


SONNE 


17.03 


A 


d 


-> 


B 


d - 


-H 


C 


Fig. 17-01 Spacing 


A 


B 


B 

A 


c(|sec) 
^ 

A(;^SeC) 


A(|sec) 
— >► 

c(^sec) 


(a) 


C(|sec) 


30 » 


A(f sec) 


30 ' 


A(^ sec) 


(b) 


c(^ sec) 


Fig. 17-02 Phasing 


17.04 


SONNE 


were to take place without any change in the phase of B, the patterns of Figures 17-03 
and 17- 04 would alternate. This is illustrated by Figure 17-05, in which the two pat- 
terns are superimposed. (Figure 17-03 contributes the dashed line, Figure 17-04 
the full line). Since Figure 17-03 would obtain for 5/6 second intervals and Figure 
17-04 for 1/6 second intervals. Figure 17-03 will be referred to as the "dash pattern" 
and Figure 17-04 as the "dot pattern". 

An observer on a craft located at P (Figures 17-03, 17-04,17-05) will receive 
stronger signals during the 5/ 6 second intervals when the dash pattern obtains than 
during the 1/ 6 second intervals corresponding to the dot pattern. He will therefore 
hear a series of dashes. An observer in the direction OQ will hear dots by similar 
reasoning. In the direction OR, both patterns yield signals of the same strength and 



Fig. 17-03 Dash pattern 


SONNE 


17.05 


a continuous signal will therefore be heard. This is referred to as an equisignal. The 
alternation of the two patterns therefore determines a number of equisignal lines 
characterized by equal signal strengths from the dot and dash patterns. These equi- 
signal lines are shown in Figure 17-05. 

In this relatively simple form, which includes no phase- sweep, the system pro- 
vides a navigational aid known as Elektra. Elektra was used by the Germans in 1940- 
41 and Sonne was developed from it. The difference between Sonne and Elektra lies 
in the slow progressive shifting of the phases of the currents in the two outer antennas. 
The effect of this slow and uniform phase shift in the A and C currents is to cause a 
rotation of the equisignal lines. Starting from the beginning of the 60- second phase- 
shifting period, the equisignals are first as shown in Figure 17-05. The equisignals 



Fig. 17-04 Dot pattern 


17.06 


SONNE 


R P Q 



Fig. 17-05 Dot and dash patterns superimposed 


SONNE 


17.0 7 


in the top half of Figure 17-05 then move clockwise, and those in the lower half 
counter-clockwise, until at the end of the 60- second period each equisignal now oc- 
cupies the position originally occupied by the adjacent equisignal to the right. At 
the left side of the dash pattern, the small lobe expands and divides; and at the right 
side the two large lobes contract into one small lobe. Corresponding changes take 
place in the dot pattern, so that at the end of the 60- second period the two patterns 
have become interchanged. 


Considering effect of these changes on an observer situated on (say) OP, 
the sequence of events will be as follows: 

(a) At the start of the cycle, dashes will be heard 

(b) Dashes decrease in contrast (5/ 6 second and 1/ 6 second signals become 
more nearly equal) until 

(c) Equisignal is heard. 

(d) After the equisignal, dots are heard, at first increasing in contrast and 
then decreasing slightly until the end of the 60- second period occurs. 


These changes are graphically represented in Figure 17-06, in which the time 
intervals are not drawn to scale. During the 3- second silent intervals there is no 
transmission from any of the antennas, and during the 56- second steady signal, only 
the center antenna is used. 





Cycle 

starts 

again 


3 sec. 
silent 


equisignal 


I sec. 
silent 


3 sec. 
silent 


Fig. 17-06 Transmission sequence 


17.08 


SONNE 


Provided the operator knows his approximate position, a knowledge of the num- 
ber of characters heard (dots or dashes) before the equisignal, and of the initial and 
final positions of the equisignal concerned, together with suitable means for interpo- 
lation, enables a line of position to be determined. 

Since in general the equisignal will not be sharply defined, and may appear to 
lastseveralseconds, the operator counts both (a) the number of dashes (or dots) heard 
be fore the equisignal and (b) the number of dots (or dashes) heard after the equisignal. 
These are added and the total subtracted from 60 (the number of characters trans- 
mitted) giving the apparent length of the equisignal. Half of this latter figure is then 
added to the (a) count so as to determine as nearly as possible the true interval be- 
fore the equisignal. For example, the observer at P might have counted as follows: 

13 dashes 
Equisignal 
41 dots 

The computation is then as follows: 

13 60 = characters transmitted 

41 M 

54 = ^haracters heard 6 = apparent length of equisignal 

13 + 2 = 16 = true number of characters before equisignal. 

Charts are provided on which position lines marked in degrees azimuth from Sonne 
stations are overprinted in color. Keys or tables are also provided by means of 
which the azimuth line of position corresponding to a given count within a given sec- 
tor can be obtained. A sketch of the central portion of such a key is shown in Figure 

17-07. Theadvantageof this procedure is that if it should become necessary to change 
the phasing of the antenna currents in order to modify the pattern for security or 
other reasons, only new keys and not new charts are required. 

It will be seen that ambiguities exist. The above count could have been ob- 
tained in any of the dash sectors of Figure 17-05. In the patterns shown, the mini- 
mum angular separation between equisignals is 9.6°. The ambiguity is thus between 
position lines whose minimum separation is 19.2°. It is therefore necessary for 
the craft to know its bearing on the Sonne station to within 9.6° or better. This may 
be done by a rough D/F measurement on the steady 56 sec. signal or the position 
may be approximately found by dead reckoning, a knowledge of the existing course 
and speed since a previous observation then being necessary. Regarding the extent 
of the ambiguity, adjacent dash sectors (or adjacent dot sectors) are separated by 
a minimum angle of about 19°. The maxinlum' error permissible in the D/F measure- 
ment used to solve the ambiguity is however only half this figure. A consideration 
of Figure 17-07 will illustrate this point. 

Suppose that a navigator makes a count of 40 dots preceding the equisignal 
and 20 dashes following it. The fact that the dots were heard first locates him in a 
dot sector. The fact that 40 dots were heard locates him at a particular azimuth with- 
in a dot sector. This azimuth is read from a key or table attached to the actual chart. 
The central portion of such a key, somewhat compressed in scale and with the finer 
markings omitted, is shown in Figure 17-07 (b). On an actual key, azimuth angles 
corresponding to the line of antennas of the particular Sonne station are read from 
the angular scale direct. In Figure 17-07 (b), it is assumed for convenience that the 
line of antennas runs from west to east, so that the central equisignal points due 
north. 


The count obtained therefore gives the navigator the following choices in line 
of position on the north side of the pattern: 

Gg, 26|°, 51°, 267^°, 326^°, 347i° 

These lines of position are shown as dashed lines in Figure 17-07 (a). 


SONNE 


17.09 




Fig. 17-07 Sector ambiguity 


Suppose further that the maximum error to be expected in the D/F observa- 
tion is the same as the angle of ambiguity, i.e., 19°. Then the situation may be repre- 
sented at (2) in Figure 17-07 (b), where the D/F reading might give an azimuth in- 
dication anywhere within the shaded range. Remembering that the D/F reading may 

give the extreme values of 347^° or 26|°, and that the navigator has only one read- 

4 o 

ing at his disposal and will take the jposition line nearest this reading, it is seen that 
he might easily choose 347^^ or 26j^ as the final reading. If however, the maximum 
error to be expected in the E/F observation is reduced to 9|^, the situation will be as 

represented in the shaded area of Figure 17-07 (a) or at (1) in Figure 17-07 (b). Even 
an extreme D/F reading will indicate to the navigator that his correct azimuth bearing 
is 6^°. The maximum allowable error in the D/F reading is therefore one half of the 

minimum angle of ambiguity. 


17.10 


SONNE 


Since observations cannot be taken closer in time than two minutes, running 
fix technique may be necessary. 

It should be noted that Sonne transmissions as here described are unmodu- 
lated (C.W.). The receiver used should therefore include a beat-frequency oscillator. 
If receivers without beat-frequency oscillators are to be used, then provision for 
modulation at the transmitter must be made. 

This concludes the general discussion of the system. Some notes on the 
geometry of the radiation pattern, and on factors affecting it, will now be given, 
together with information as to the transmitting equipment used, a discussion of 
transmitter errors and tolerances, and an outline of a proposed two- antenna system. 



i^ = + A sin (ct)t + <^) ^2 “ ® 

i 3 = + A sin (cot - 0) 

B/A = p spacing = nX 

77 

(f> = (gQ)t radians, t measured in seconds 

Field strength at a distant point P of azimuth angle 6 
E = k.cos cot. [B ± 2A sin (</> - 277 n sin 0)] 
where k is a constant determined by the distance of P from 
the array and by propagation conditions . 

The radiation pattern of Sonne 

Many arrangements yield multi- lobed patterns of the type described. Two 
basic arrangements are here discussed: 

(1) three antennas, equally spaced, equal currents in the two outer antennas, phased 
as indicated in Figure 17-03. This is the system used by the Germans. 

(2) possible two-antenna arrays yielding the same results as (1). 

Three antenna Sonne 

Let the spacing d = nX. Note that n need not be integral. The currents in the 
three antennas are i^^ = + A sin (cot + <^); i 2 = B cos cot; i 3 = + A sin (cot -^). (See Fig- 
ure 17-08). The + and - signs in ii and the - and + signs in i 2 yield the dash and dot 
patterns quoted previously if the upper sign obtains for 5/6 sec. and the lower sign 
for 1/ 6 sec. The phase- shift </> in the outer antennas is given by <^ = (^)t radians where 
t is measured in seconds from the start of the phase- shifting period. The ratio of the 
current amplitudes is denoted by B/A = p. 


SONNE 


17.11 


There are two physical variables: I 

(1) n (2) p \ 

The following factors relating to the pattern are of interest: 

(1) total number of equisignals in the pattern. 

(2) minimum angle of ambiguity. One half of this is the maximum tolerable error 
in the D/F measurement required to solve the ambiguity of sector, as already 
noted. It should be as large as possible, that is, a value of 360® would indicate 
complete certainty of sector. 

(3) The ratio = p- For most economical use of a given radiated 

power to obtain large useful range, should be as near unity as possible. This 
ideal condition is not realizeable in practice with either two- or three-antenna 
arrays. 

(4) Absolute field strength at equisignal. This determines the maximum useful 
range. 

(5) Rate of change of field strength with azimuth in the region of an equisignal. This 
is a factor in determining the discrimination with which a bearing may be ob- 
served (that is, the theoretical accuracy obtainable). 

(6) The number of db. difference in level between the dot and dash signals for azi- 
muth departure from an equisignal. 

The way in which these five factors depend on the variables (current ampli- 
tude, antenna spacing) is summarized in Table 17-01. 


Table 17-01 



Current Amplitudes 

Spacing 

total number of equisignals 
in pattern (1) 

no influence 

equals 8n 

minimum ancle of ambiguity (2) 

no influence 

decreases as n increas- 
es (equal to 2 sin"^l/2n) 

_ equisignal field (3) 

1 maximum field 

p =-^ 

^ P+2 

no influence 

absolute field strength at equi- 
signal (4) 

proportional to 

B only 

no influence 

rate of change of field strength 
with azimuth (dE /d0) at equi- 
signal (5) 

proportional to 

A only 

proportional to n 

also proportional to cos 0 

db. difference ^^^between dot and 
dash signals per degree azimuth 
departure from equisignal = a 

, 1+ 0.22 (n/p) cos 0 

20 log jQ j _ 0 22 (n/p) cos 0 


sin Og ^ ^ /_t_ + m V , where m is any inte- 
2n 60 

ger including zero 


Equation determining equisignal 
positions 


17.12 


SONNE 


Notes on Table 17-01 

1. If the.spacingd is an integral number of half-wavelengths (that is, if n= 0.5, 1,0, 
1.5, 2.0, 2.5, etc.), equisignals occur at 0 =± 90®, and by symmetry the number 
of equisignals in the pattern is always even, whatever the value of n. If n is some 
other number (e.g. 2.3), the number of equisignals will be the next even number 
above 8n (e.g 20). This is not however, the number of useful equisignals. In the 
pattern drawn, it will be noted that the accuracy will be very poor near 0 = ± 90®, 
so that a safe rule for the number of useful equisignals is 8n - 2. 

2. The angle of ambiguity is the angle between the equisignal concerned and the next 
but one adjacent to it. This is a minimum (requiring the greatest accuracy of 
approximate D/F observation) at 0 = 0^. (Equisignals occur at 0 = 0^ and 0 = 
1800 in all patterns covered by this analysis). Since equisignals occur where sin 
(27rn sin 9) = 0, the angle concerned is 2 sin “^l/2n for the first pair of equisignals 
either side of 0 = OO. In the pattern shown in Figure 17-05, n= 3 and p = 4, so 
that there are twenty-four equisignals in all, occurring at the following approxi- 

mate values of 0: 0°,^ 9l°, ±19^°, ±30°, ±41?°, ±56^°, ±90°, ±123i°, ±138^°, 

^2 4 ^ 2 4 

±150°, ± 160l°, ± 170^°, 1800. The minimum angle of ambiguity is therefore about 

19°. 

3. With the phase relationships here assumed, the pattern may be thought of as made 
up of (a) a uniform component due to the central antenna, (b) a component due to the 
two outer antennas which varies in phase and in magnitude as 0 is varied, causing 
increments and decrements to the uniform component. Thus in the example given 
B = 4A (p = 4) and the ratios of maximum field, equisignal field and minimum field 
are as (B + 2A) : B : (B - 2A), that is, as 6 : 4 : 2 or as 3 : 2 : 1 . 

4. Since at the equisignals the fields due to the two outer antennas exactly cancel, 
the equisignal field depends only on that due to the center antenna. 

5. Analysis shows that dE in the region of an equisignal is given by 4k Tin A cos 0 

units per degree azimuth where k is a propagation constant depending on the dis- 
tance of the observer from the transmitter. This gives the slope of either the 
dot or the dash pattern at the equisignal. 

6. Of more interest than is the number of db. difference in signal strength between 


the dashanddot patterns per degree azimuth departure from equisignal. This is 

dE 


E + 


d0 


E ^ 
^■d0 


This reduces too? (db) = 20 log 


, where E is the equisignal field (of magnitude 

per degree and 
1 + 0.22 ?cos 0 


given by a (db) = 20 log 
kB) and is measured in units per degree and is evaluated at the equisignal 


1 -0.22- cos 0 


decibels per degree azimuth. 


This expression is characteristic only of the geometry of the radiation pattern 
and does not depend on propagation lactors or on distance irom the array. Apply- 
ing this result to the example previously used (n= 3, p= 4), the maximum change 
in signal strength per degree is obtained at 0 = 0<^, under which conditions (db)= 

20 log 1 ^ ^ ^ 2.9 decibels per degree azimuth. If it be assumed that an 

- 0.165J 


operator under average conditions can detect a 1 db. change in audible signal 
level, then the position of the central equisignal (0 = 0^) can be observed direct- 
ly to about± 1/3° azimuth. However, since the technique of counting dots and 
dashes both before and after the equisignal is used, greater discrimination than 
this may be obtained, and the limiting factor under these conditions is not in prac- 
tice the change in signal strength per degree azimuth but is concerned with the 


SONNE 


17.13 


number of characters transmitted per minute. This is further considered later 
under the heading Theoretical Accuracy. 

In general it is to be noted that the change in the signal strength per de- 
gree azimuth at equisignals varies as follows: 

Maximum at 6 = 0, decreasing to zero at 0 = + 90^ 

Increases as n is increased. 

Decreases as p is increased. 

Two- antenna Sonne 

It has been pointed out that similar results may be had if only two antennas 
are used. The spacing between antennas is n'X and the currents are ii = A sin (uiMf)) 
and i 2 = i B cos wt (Figure 17-0*9), and0 = The equisignals occur at azimuth 

angles where the magnitudes of the dash and dot fields are equal, that is, where sin 
(<f) - 27rn' sin 0) = 0. Since this is the same equation as that which determines equi- 
signals in the three-antenna pattern, the equisignals will occur at the same azimuth 
angles if n = n' , and the 2 antenna pattern will be similar to the 3-antenna pattern. 



11 = A sin (a)t + 0) 

1 2 = ± B cos cot 

E- = ratio of amplitudes = p' 

A 

spacing = n* X 

(f) =(^) t radians, t measured in seconds 

Field streng th at a distant point P of azimuth a ngle 0 
is E = kAV 1 + p' 2 + 2p' sin (<^ - 27rn' sin 0) where 
k is a constant representing the distance of point P 
from the array and the propagation conditions. 

In Table 17-02 the features of the pattern and their dependence on physical 
variables (p' = B/A and n' = c^/X) are listed. 


17.14 


SONNE 


Table 17-02 



Current Amplitudes 

Spacing 

total number of equisignals 
in pattern (1) 

no influence 

equals 8n' (or next 
higher even number 
if 8n' is not even)(^) 

minimum angle of ambi- 
guity (2) 

no influence 

decreases as n' in- 
creases (=2 sin l/2n') 

p' _ (equisignal field) 

' ( maximum field) 

o’ 

r 1 -H p 

no influence 

absolute field strength 
at equisignal 

a = k\/ 

kA\/ 1 +p'^ 

no influence 

Rate of change of field 
strength w^ azimuth at 
equisignal ^ ' 

proportional to 

Ad' AB 

= vpTP 

proportional to n' 

also proportional to cos 0 

db difference ^ ' between 
dot and dash patterns per 
degree azimuth departure 
from equisignal = ol ' 

a ' = 20 log jQ 

1 +p' 2 + 0.11 p'n'cos 01 

1 + p' 2 _ 0.11 p' n' cos 01 


equation determining equi- sin ©e = (-^ ± m), where m is any inte- 

signal positions including zero. 


Notes on Table 17-02 

1. Equisignal considerations follow from the same equation as for the three-antenna 
case. The equisignals at 0 = i 90^ (in the case where n' A. is an integral number 
of half wavelengths) are not useful and in this case the number of useful equisig- 
nals will be 8n^ - 2. 

2,3 The same considerations apply as in the three-antenna case. 

4. Since at the equisignals the magnitude of the field is unaffected by the choice of 
+B or - B, equisignals only occur at positions such that the received A and B 
signals are in phase quadrature. 

5. ^ in the region of an equisignal is given by 


c ^ units per radian or n cose degree azimuth 

Vi +p'2 Vl +p'2 

where k is constant for any one position of the observer. 

6. Analysis shows that, for any given value of n‘ and cos 0,0: ‘ will have its maxi- 
mum value when p‘ = 1, that is, when the amplitudes of the two antenna currents 
are equal. Further, if p‘ =1, and n‘ = 3, and cos 0 = l,a'=2.9db per degree 
azimuth. Comparing this with the result for the 3- antenna Sonne quoted on 
page 17.12, it is seen that the two- and three-antenna Sonnen give very closely 
corresponding pattern features. 


SONNE 


17.15 


Theoretical Accuracy of the System 

Assuming no errors in phasing at the antennas and no errors due to propaga- 
tion conditions, the accuracy of a Sonne bearing depends upon the following: 

(1) The rate at which signal strength changes with azimuth in the region of the equi- 
signal. This has been discussed above. 

(2) The effect of the counting technique used, which is to reduce (we hope) any errors 
due to a slow rate of change in (1) above. 

(3) The number of characters transmitted per minute. This is assumed to be 60. 

It is doubtful whether this number could be increased without danger of errors 
in the actual count. The cycle might be lengthened to (say) 90 characters trans- 
mitted in li minutes, but this would involve either increasing the interval be- 

tween possible readings, or decreasing the length of the steady transmission 
available for D/F observations. 

Three points may be noted with regard to the sizes of the errors due to 
these causes: 

(a) Since the count cannot in any case be made to within less than one character, 
we are limited to an accuracy determined by the angle through which the 
equisignal sweeps in 1 second. For the central sectors in the example given 
in Figure 17-05, this is approximately 1/6 degree azimuth. In the same ex- 
ample, the error due to a one- decibel limit of dot- dash discrimination, + 
1/3 degree azimuth, when modified by the results of the counting technique, 
probably amounts to less than this figure so that factor (3) above is likely 
to be the limiting one. These figures would of course be modified if the con- 
stants p and n had been given different values. 

(b) Errors due to factor (1) above increase as 0 increases, being inversely pro- 
portional to cos 0 . However , the increase will not be large until 0 approaches 
75 - 80®, in which zone the utility of the system is not considered great. 

(c) Errors due to factor (3) above also increase as 0 increases, since the angu- 
lar separation of equisignals is larger at the sides of the pattern, causing 
the angular speed of movement of the equisignals to increase also at larger 
values of 0. The constants of the German system appear to have been well- 

chosen since the errors due to both (1) and (3) are of about the same magni- 
tude, and since a given Sonne beacon has a reasonable coverage arc over 
which the maximum theoretical error will not exceed twice the value at 0= 
0 ®. 

German Transmitting and Phasing Equipment 

Details have recently become available concerning the standard German phas- 
ing equipment. A simplified circuit diagram is shown in Figure 17-10. The RF pow- 
er from the transmitter (1.5 kw) is divided between the center antenna, which re- 
ceives 8/9 of the total power (if B= 4A), and the end antennas. Switch S and the di- 
vided primary of allow reversal at 5/6 and 1/6 second intervals. The loop fed 
from the secondary of T^ is tuned to resonance, as is the next loop including the 
primary of T2. P is a phase shifter which is preset and controls the exact phasing 
of the end antenna currents with respect to the current in the center antenna. The 
loop fed by the secondary of T2 is also tuned to resonance and includes the rotor 
coil (1) of the goniometer G. The two stator coils (2,3) are mutually at right angles. 
The voltages induced in them have the same phase, but are of magnitudes proportional 
respectively to the sine and cosine of the angle through which the rotor has turned. 
Thus when coil 1 is parallel to coil 2, E2 will be a maximum and E3 zero. As the 
rotor turns, E2 decreases and E3 increases, these voltages remaining in phase. The 
primary of T3 being tuned to resonance, the voltage E^ induced in its secondary is 
in quadrature with E3. 


17.16 


SONNE 


The voltages E 2 and E4, which are therefore in quadrature, are applied at ab 
and cd respectively to the condenser network Q which functions as a mixer. All 
eight condensers are of equal capacitance (about 1030^f). Outputs are taken at ef 
(E5) and gh (E6). E5 is therefore proportional to the vector sum of E2 and E4, and 
E0 to their vector difference. These two output voltages, developed in loops which 
are tuned to resonance, are applied to the 600- ohm open- wire transmission lines 
which feed the two end antennas. 

The phase- relationships at various times in the phase- shifting cycle are 
represented in Figure 17-11. At t = 0, E2 = 0 and E^ has its maximum value, de- 
noted by E. E5 and E0 are therefore equal to E in magnitude and are opposite in 
phase. At t = 15 sec, coil 1 has rotated through 45^, E2 and E4 are equal in magni- 
tude and are 0.707 E. E5 and E0 have therefore remained equal to E in magnitude, 
but have shifted their phases by 45^ in opposite directions. At t = 30 sec, E2 = E 
and E4 = 0, so that E5 and E0 are now in phase. At t = 45 sec, E2= 0.707E and E4 
has changed sign. It will be seen that the two output voltages are always of equal 
magnitude and that their phase shifts due to the rotation of the goniometer are equal 
and opposite. 

The effect of the keying at S is to reverse all phases, including that of the two 
output voltages. During the second 180^ of rotation of coil 1, no power is supplied 
to the phase- shifting system (56 sec. omni-directional signal from center antenna 
alone), and at the end of the two minute cycle the phases are again in their original 
relationships . 

Circuits of monitoring and protective equipment are not given here. 

Two- antenna Sonne 

Figure 17-12 shows a block diagram of a scheme proposed by Commander 
E.N. Dingley, Jr., U.S.N.R. for the phase-shifting and keying of a two-antenna Sonne. 

The crystal oscillator (1) of frequency fj drives a phase- shifter of the three- 
phase capacitor- goniometer type (2). The output from the phase-shifter is continu- 
ously variable in phase and is applied to a mixer (6) by way of a buffer amplifier (3). 
The other input to the mixer is from a second crystal oscillator (4) of frequency f 2, 

using a buffer amplifier (5). A band-pass filter (7) selects the output component at 
the sum frequency (fj +f2). A relay (8) and adjustable attenuator (9) are inserted 
between this output and the transmission line to one of the two antennas. At the 
antenna mast, the frequency is divided by two (10) and the resulting signals drive 
a power amplifier (not shown) and thence the antenna (11). 

The second antenna is fed in the same way, except that the input to the buffer 
amplifier (16) is taken direct from the crystal oscillator (1) and not from the out- 
put of the phase shifter. 

The outputs from the two antennas are therefore of frequency ^.nd 

the signals passed over the transmission lines are of twice this frequency. By this 
means it is expected that the effect of radiation from the transmission lines upon the 
main radiation pattern will be reduced. Furthermore, the phasing of antenna no. 1 
with respect to antenna no. 2 is 0/ 2, if 0 is the phase -shift introduced by the phase 
shifter (2). This makes it possible to use one complete rqtation of the phase shifter 
(0 = 360^) to produce a phase shift of 180® as required in the transmission from 
antenna no. 1. 

Keying and phase- shifting are accomplished by a synchronous motor which 


mpjt 


SONNE 


17.17 



-= < 

C 

O <l> 
O c 
o o 


— O 

< I 

O a> 



H" 


Hi- 



TT^RT JW JW" 


Hi- 


(/) , 


Fig. 17-10 German phase shifting circuits 


17.18 


SONNE 



E5=E6=E2=E(dash) 


E4 = 0 


^5=^6=E2=E(D0T) 



t= 30 sec. 


t= 45 sec. 


£ 5 = E 2 +E 4 
E0= Eg-E^ 


Fig. 17-11 Vector diagram illustrating German method of phasing 


SONNE 


17.19 




(\j 

‘f- 

+ 




Hi* 


or 



tt: LJ 


q : 

UJ li_ 

t ^ 

© 

UJ 

Ll. 

u_ 

3 Q- 


Z) 

CD 2 


m 

< 



Ql 



q: 

_i o 




< 




H < 



(/) -J 
> -1 

o 




o (O 



o (/) 

o 



o 


.<NJ 


(\i 


© 


>- 


o 

q: 

2 

UJ 

UJ 

Q 

3 

O 

> 

UJ 

o 

tr 


u. 



CVJ 

M- 

+ 


UJ 

(T 

3 

O 

GD 

h- 

< 

< 

}— 

3 

CO 

Z 

3 

UJ 

“3 

h- 

Q 

h- 

< 

< 










RELAY 


MIXER 

© 

RELAY 



CO 





-1- 




ftD 


Fig. 17-12 Block diagram, two-antenna Sonne 



17.20 


SONNE 


is made to operate the phase shifter (2) through reduction and differential gearing, 
and also operates relays (8) and (19). The following cycle is proposed: 


0-60 

sec. 

both relays closed 

60-64 

sec. 

both relays open 

64-116 

sec. 

relay (8) open, (19) closed 

116-120 

sec. 

both relays open 

120-180 

sec. 

both relays closed 

180-184 

sec. 

both relays open 

184-236 

sec. 

relay (8) closed, (19) open 

236-240 

sec. 

both relays open 


repeat cycle 


At all times the phase- shifter (2) rotates at a uniform angular speed of 360^ 
per minute. It is assumed that the phase- shifter is so constructed that 1^ of rotation 
produces 1^ of phase shift at all angles. Further, there is a phase- reversing relay 
(which reverses the polarity of the leads) connected between the output of the frequency- 
divider and the input to the transmitter at antenna no. 2. This relay is operated 
(5/6 sec. in one position and 1/ 6 sec. in the other) over a control line from a contact 
controlled by the same synchronous motor. 

It will be noted that the omnidirectional signal radiated during the 52 second 
period is transmitted alternately by the two antennas. This permits the current 
amplitudes to be checked, by means of a monitoring receiver fitted with a calibrated 
outputmeter and located exactly mid- way between the two antennas. This also allows 
the monitoring operator to check the time-position in the cycle of the equisignal. If 
this is not correct, or if it is to be altered, the setting of the rotor of the phase- 
shifter (2) relative to the driving shaft is changed by means of the differential gears. 
The uniformity of the change of phase with angle of rotation is checked by the record- 
er (15) operated by the mixer (13) and low-pass filter (14). This is arranged to give 
a record (on a moving tape) of the cosine of the phase angle 0. This record is com- 
pared with a standard cosine curve. 

Another advantage claimed for this arrangement is that the crystal oscillator 

(1) and phase shifter (2) are standard equipment for all Sonne stations, the oscillator 
being of standard frequency and the phase shifter of standard design. Sonnen of 
different frequencies are accomodated by choosing different values for f2, the fre- 
quency of the oscillator (4). 

Transmitter errors and tolerances 

Errors in the magnitudes or phasing of the antenna currents will distort the 
Sonne radiation pattern. The results of this may be classified under three headings: 

(1) Actual shift in the angular positions of the equisignals at the start of the phase- 
shifting cycle. The navigator determines his apparent line of position by refer- 
ence to a key or table on which the azimuth positions of equisignal lines at the 
start of the cycle are shown. If the equisignals do not lie in the marked posi- 
tions, there will be a corresponding error in the line of position obtained. This 
error will be of the constant type, predictable if the nature and extent of its 
causes are known. 

(2) Loss of discrimination. If the change in signal strength per degree azimuth 
angular shift from an equisignal becomes smaller for any reason, the theoreti- 
cal precision of a reading decreases. Such errors will be of the random type, 
averaging out among a very large number of readings. 

(3) Loss of uniformity in the speed at which equisignals sweep in azimuth during 
the phase- shifting cycle. To obtain a line of position, the navigator refers to 
the table or key which shows the relation between the number of dots (or dashes) 


SONNE 


17.21 


preceding the equisignal and the corresponding bearing on the Sonne transmitter 
withinthe sector on the chart identified by D/F or dead reckoning. This table or 
key is based on the fact that sin (-^ t - 27rn sin 0) = 0 at equisignals. The relation 
between t (time in seconds from start of phase- shifting period) and 0 (azimuth of 
equisignal at any instant) is therefore sinusoidal. Any errors in phasing which 
result in a departure from this sinusoidal relationship amount to errors in posi- 
tion-line interpolation in time within the sector used, even though the initial and 
final values of 0 which determine the sector may themselves be correct. 

The most serious error is that identified in (1) above. In general, most effects 
which produce appreciable errors of the second and third types will also produce 
much more serious errors of the first type. A number of possible departures from 
ideal conditions at the transmitter will now be mentioned for both two- and three- 
antennaSonnen, together with their effect (if any) on the angular positions of the equi- 
signals at the start of the phase- shifting period. Errors due to propagation conditions 
are not peculiar to the Sonne system and are separately discussed elsewhere (see 
Section 1). Itis to be noted that each possible cause of error is considered separately. 
The simultaneous consideration of two or more sources of error cannot be general- 
ized, and the number of possibilities is therefore too large for inclusion here. A 
factor which by itself does not produce any equisignal shift may operate to increase 
or decrease errors due to some other factor. Twofactors which separately produce no 
appreciable error may give rise to considerable errors when combined simultaneously. 

(a) Three-antenna Sonne 

1. Currents in the outer antennas are not equal in magnitude. This condition is repre- 
sented, 20 seconds after the start of the keying cycle, in Figure 17-13 (a), which 
shows the current vectors at the transmitter. Figure 17-13 (b) shows the compon- 
ents of the received field at a point whose azimuth angle corresponds to the first 
equisignal, at this particular instant. The effect of placing the receiver at some 
azimuthother than zero is that the distances from the receiver to the three anten- 
nas are not equal. This is illustrated in Figure 17-13 (d). Considering the B 
component of the received field (Figure 17-13 (b)) as phase reference it will be 
seen that the A component is made to lag by an amount proportional to Ap and the 
C component to lead by the same amount since the antenna spacings are equal. 

If 0 is such that the lag and lead thereby introduced in the received field components 
exactly compensates for the lead and lag introduced by phase shifting at the trans- 
mitter, the field components at the receiver will be as shown in Figure 17-13 (b) 
for a dash, and in Figure 17-13 (c) for a dot. It is seen that the resultant field R 
is equal in magnitude for the dot and dash fields, although the dot and dash phases 
are not the same. 

The equality in magnitude of the dot and dash fields is not affected by in- 
equalities of the A and C antenna currents. Therefore equisignals are observed 
at the same positions and times no matter whether the A and C currents are equal 
or not, and no errors result. If the A and C currents had been equal, the phases 
of the received fields would also have been identical. The relative magnitudes of 
the current vectors in Figure 17-13 are purposely not drawn to scale. 

2. The phase- shifts of the currents in the outer antennas are not equal. This is illus- 

trated in Figure 17-14 (a), in which the phase- shifts are and <^2 the A and C 
currents respectively. To obtain an equisignal, the observer must shift to an azi- 
muth such that the lag and lead thereby introduced into the A and C field compo- 
nents is This is illustrated in Figure 17-14 (b) (dash) and Figure 17-14 

(c) (dot). Since the A and C fields cancel, an equisignal is obtained. This equi- 
signal would have occupied a slightly different position if had been equal to </) 2 . 
The difference is small if (/>! and <^>2 are not too unequal. A difference of 10° between 
</)j and (^2 shifts the position of the central equisignal by only 16' , if n = 3 (see 

Figure 17-05). 


17.22 


SONNE 


B Rdash 





(c) (d) 


Fig. 17-13 Vector diagrams illustrating incorrect amplitude 


SONNE 


17.23 


B ^“f^dash 





(c) 

Fig. 17-14 Vector diagrams illustrating incorrect phasing 


17.24 


SONNE 


3. Change in frequency or in antenna spacing. These are equivalent as far as the 
radiated pattern is concerned, since the pattern geometry depends on nX. A change 
in frequency will of course have profound effects if the antenna tuning circuits 
(andany other tuned circuits) have not been realigned, but it is presumed that this 
has been done and that the frequency change was made for some definite reason — 
e.g. to avoid jamming. Small errors in siting the transmitting antennas are also 
covered under this heading. 

Analysis shows that the center equisignal will not be shifted at all by such 
a change or error, and the others slightly. The shift increases as 0 increases, 
being proportional to tan 0. Taking n = 3 and the least favorable case (fifth equi- 
signal at 6 = 56^^), this equisignal is shifted by only or by 5' for a 0.1 7o fre- 
quency change or a O.lVo error in antenna spacing. 

4. The phases of the A and C antenna currents are correct, but that of the B antenna 
current is incorrect. This is equivalent to inequality of phase shift of the outer 
antenna currents, discussed under (2) above. The central equisignal will be shifted 
by 16' if the phasing of the B antenna current is incorrect by 10^. Other equi- 

signals are shifted by proportional amounts, the shift being proportional to . 

COS u 

5. The magnitude of the B antenna current, or of the A and C antenna currents, de- 
parts from its assigned value. This amounts to a change in the value of p = 

A 

and it will be noted from Table 17-01 that the number and disposition of the equi- 
signals is not affected. 

6. The phase- shift applied to the outer antenna currents is not linear with time. The 
effect of this error is that the actual phasing of the transmitted signals at some 
instant t seconds after the start of the phasing cycle is that which should have 
existed at a different instant t' seconds after the start of the phasing cycle. This 
is the same effect as would be produced by an error in the counting of dots and 
dashes . The error in azimuth angle thereby introduced is a minimum for the 
central equisignal at 0 = 0^, for which a phase departure of 10° from the proper 
value corresponds to an error of 3 characters in the count, which in turn produces 
an error of 29' in the line of position obtained. For other equisignals the error 

is larger, being proportional to — i—. 

COS0 

(b) Two- antenna Sonne 

1 . The magnitudes of the currents in the two antennas depart from their assigned 
values. This is a change in the value of p = B/A. From Table 17-02, it is seen 
that although such an error will affect the sharpness of discrimination, the num- 
ber and position of the equisignals will not be affected. 

2. The phase of the B antenna current during a dot (or dash) period departs from 
its assigned value. This results in a shift of the equisignals. If the phase is in 
error by A</) degrees, the equisignals are shifted by an amount A0 (degree^ = 

For the central equisignal and an antenna spacing of three wavelengths 

a 10® phasing error produces an equisignal shift of 32' . Other t^quisignals will be 

shifted further, the shift being proportional to — 

’ cos 0 

3. Change infrequency or antenna spacing. Assuming the phasing of all currents to 
remain correct, and considering only the change in nX as it affects the radiated 
pattern, the effect of this change is precisely the same as in the three- antenna 
case. That is, a 0.1 1 change in frequency or in spacing shifts the fifth equi- 
signal by 5' for n = 3, the central equisignal not at all. 


SONNE 


17.25 


4. The initial phase of the A antenna current departs from its assigned value. That 
is, 0 at t = 0. The effect of such an error with the 2-antenna Sonne is the 
same as an inaccuracy in the phasing of the dot or dash B- antenna current, dis- 
cussed under (2) above. 

5. The shift in phase during the cycle is not linear with time. The same remarks 
apply here as in the three-antenna case. (See (a), 6 above.) 

Comparison of two- and three-antenna Sonnen 

1. Siting . The site of a Sonne station should be free from directional non- uniformities 
over a considerable area, and should also be flat. Other conditions being equal, 
a two-antenna Sonne should be easier to site than a three-antenna Sonne. 

2. Cost . In any low-frequency system, the outlay on antenna- towers and on the 
ground system represents a sizeable part of the capital cost. This consideration 
therefore favors a two-antenna design. 

3. Power rating of transmitters. Using the numerical values for spacing and 
current ratio already taken as typical (n = 3, p = 4 for three-antenna Sonne and 
n' = 3, p‘ =1 for two-antenna Sonne) the equisignal field strength at the same 
distance in the two cases is proportional to B for the three- antenna station and 
to ^/2B' for the two-antenna station. If these equisignal field strengths are to be 
equal B' should equal 0.707 B. The total power radiated is proportional to 
b2 2A^ = B^ for the three-antenna case and to 2B' ^ = B^ for the two-anten- 
na case. Therefore, if the two-antenna and three-antenna designs are to produce 
equal equisignal field strengths at equal distances, the three- antenna Sonne 
must radiate 12^^ more power than the two-antenna Sonne. 

4. Power- Handling capacity of phase shifter andkeyer. With the two- antenna design, 
one- half of the total power must be keyed (for dot and dash patterns) and phase- 
shifted (for pattern rotation), if the antenna currents are equal and if the phase- 
shifting and keying is done at high power-level. Under the same conditions, only 
one-ninth of the power must be keyed and phase- shifted with the three-antenna 
design. If high-level keying and phase- shifting are used, this point appears to be 
a conclusive argument in favor of three antennas as opposed to two, and probably 
represents the main answer to the question of why the Germans used the three- 
antenna design. 

5. Key clicks. With the three -antenna phasing and keying arrangement used by the 
Germans, there is no change of RF phase at equisignals between the dot and 
dash fields, since the equisignal field is due to the steady current in the center 
antenna alone. With the two- antenna design, the RF field changes phase by 90® 
at the equisignal if the antenna currents are of equal magnitude. Using a receiver 
containing high-Q RF circuits, key clicks would probably be more severe with 
two-antenna transmission for this reason. 

6. Sharpness of equisignal discrimination. Using equal currents in the two- antenna 
Sonne, a current ratio p = 4 with the three-antenna Sonne and equal antenna spac-> 
ings of three wavelengths, it has been already noted that equal discrimination is 
obtained in the two cases. 

7. Susceptibility to errors in the phasing and keying circuits. From the considera- 
tions of transmitter tolerances and errors already given, it may be seen that the 
two- antenna system is slightly more susceptible to equisignal shifts than is the 
three-antenna system. Exact comparison under this heading will of course de- 
pend on the circuits used to realize the required results. 


17.26 


SONNE 


Bibliography 




Identification 

Classification 

Title 

Issued by 

JEIA 8416 

Secret 

Aids to Navigation Memo no. 8 
Consol plotting chart, Faeroes 

Coastal Com- 
mand 

JEIA 6826 

Secret 

German Long-range Navigational 
system: Notes on Sonne Naviga- 
tional beacon system: Consol as 
an aid to Navigation 

Intelligence Div. 
C.N.O. 

G "^0984 

Secret 

Observations of the German 

Sonne system 

Toronto Confer- 
ence 

JEIA 7072 

Secret 

Instructions for plotting radio 
fixes by the G.A.F. Radiobeacons 
"Elektra Sonne". 

Intelligence 
branch, O.C.S.O. 

S 67-5 

Secret 

M.I.T. comments on the Sonne 
(Consol) Navigation System 

N.L.O. Div. 14 
NDRC 

JEIA 7080 

Secret 

Investigation of Loran, Sonne 
and Decca navigational aids. 

Intelligence Div. 
C.N.O. 

S.935-17 

Secret 

Sonne 

BUSHIPS 

WA.4099-3 

Secret 

An investigation to find a suitable 
method of checking the stability 
of the position- line given by Sonne 

P.O.E.D., Lon- 
don 

JEIA 7944 

Secret 

Theoretical comparison of the two- 
and three- aerial Sonne systems 

. O.C.S.O. 
Washington 

WA.4312-3 

Secret 

German Sonne navigational air 
radio station, investigation of, 
12/14/44 

C.I.O.S. 

SHAEF 

JEIA 2809 

Secret 

Consol Range and Accuracy 

Trials 

Intelligence Div. 
CNO 

VA 2/5260 

Secret 

Circuit diagram of German phas- 
ing equipment for Sonne trans- 
mitters. 

Watson Labs 

Report no. 9 

Secret 

Interim report on the Sonne 
(Consol) Navigation System 

Watson Labs 

JEIA 10908 

Confidential 

'Elektra-Sonne" (Translation of 
Lorenz description and Operat- 

Air Ministry, 
London 


ing Instructions for Sonne 8 
HF-Rack 111) 


BENDIX 


18.01 


Bendix Automatic Position- Plotter 


Type of system 
Azimuth 

Useful range and coverage area 

Depends on power of ground beacons, also on height of navigated craft. 
Accuracy 

The accuracy of a fix obtained by this system is limited by the accuracy of 
the automatic direction-finders used. A fairly conservative estimate would be ± 3^ 
azimuth in either of the lines of position which yield the fix. 

Type of presentation 

Visual. Continuous and automatic indication of position is given visually on 
a chart. 

Operating skill required 

(a) Craft: Two automatic direction-finders of standard type, flux-gate com- 
pass, specialized computer (automatic in operation), plotting board with special 
attachments, (b) Ground: Two beacons requiredfor a fix. For coverage of a large 
area, a number of beacons would be required. 

Radio-frequency spectrum allotments necessary 

Each beacon transmits on a different frequency. The automatic direction- 
finder as at present used covers the frequency range 100 kcps - 1600 kcps. Unless 
other information were to be transmitted by the beacons, no modulation would be 
necessary and each beacon would require only a single frequency. 

Present status 

The specialized computer and plotting board attachments are now being de- 
veloped experimentally and a working model is expected to be in operation at the 
Bendix Company' s development laboratory in a few months. Flux-gate compasses 
and automatic direction-finders are standard equipment already. 

Principle of Operation 

Referring to Figure 18-01, suppose that the craft is at R and the two ground 
beacons at P and Q. The craft is headed in the direction RH, and RN represents 
magnetic north. The flux-gate compass on the craft provides continuous readings 
of the angle A between the heading and magnetic north. The two automatic direction- 
finders give continuous indications of Band C, the angles of the beacons Q and P with 
respectto the heading. The computer therefore receives three channels of informa- 
tion: A, B and C. It performs two functions: 

(a) By means of differential synchros, the angles and <^2 ^^e computed. These 
are the magnetic bearings of the beacons with respect to magnetic north. The 
magnetic deviation is set into the computer as a constant, so that from these 
angles the true bearings are obtained. In Figure 18-01, no distinction has been 
made between true and magnetic north, for the sake of simplicity. 

(b) If the positions of P, Q and R are specified by rectangular coordinates with res- 
pect to axes OX and OY, then it will be realized that by the application of trigo- 
nometry, the coordinates x and y of the craft can be computed in terms of the 
constants X 2 X 2 yiy 2 and the observed angles and This the computer does. 


18.02 


BENDIX 


The plotting board has a small carriage ("bug") to which may be attached a 
pointer, source of lighter recording pen. This carriage is supported on, and moved 
by, a framework running on two long threaded lead screws parallel to OX and OY. 
The lead screws are rotated by means of small motors driven by suitable amplifiers, 
into which are fed error voltages which represent the differences between the out- 
put of the computer (x, y) and "position" voltages which are proportional to the 
coordinate distances by which the carriage is displaced from the origin of coordi- 
nates. These "position" voltages are obtained from long wire- wound potentiometers 
supported below and parallel to the threaded lead screws, contact springs being 
mounted on the screw heads which propel the carriage in the x and y directions. 

The block diagram of Figure 18-02 illustrates the method by which these re- 
sults are obtained. 

Referring to Figure 18-02, the two automatic direction-finders provide data 
as to the angles B and C in Figure 18-01. The flux-gate compass gives the angle A. 
By means of differential synchros, the angles a and p are fed to the computer, in the 
form of physical rotations. 



Fig. 18-01 Angular relations 


BENDIX 


18.03 


It may be shown that the coordinates x, y of the craft are given by the follow- 
ing equations, in which the quantities ^ significance indi- 

cated in Figure 18-01. 


sin p 

1 sin (a - 0) 


[ (x^ - X 2 ) cos Q? + (y 2 - y^) sin o: ] 


( 1 ) 


y = yi -^ sinW- py ^ ' ’'2) « + (y2 - yi) sin « ] (2) 

Assuming that voltages proportional to x and y could be generated, and that 
voltages proportional to x' and y' , representing the actual position of the cursor at 
any time, are obtained from the long wire- wound x and y potentiometers, then x' - 
X and y' - y would represent error voltages which, when suitably applied to the x 
and y driving motors, would correctly position the cursor so that x' - x = y' - y = 
0. However, the operation of multiplying by 1/sin (a - /3) cannot be adequately per- 
formed since this quantity varies between 1 and 00 . Therefore the above equations 
are multiplied by sin (oi - /3), giving 

X sin (q? - /3) = x^ sin (o? - /3) + sin [ (x^ - X 2 ) cos a + {y 2 - y^) sin a ] (3) 

y sin (a - ^) = y^ sin (o? - ^) + cos ^ [ (x^ - X 2 ) cos oi + (y 2 - y^) sin a ] (4) 



Fig. 18-02 Block Diagram 








18.04 


BENDIX 


This means that the voltages x' and y’ must also be multiplied by sin (o? - /3). The 
error voltages which drive the x and y motors will then be given by 

x' sin (a - - [x^ sin (o? - /3) + sin ^ [ (x^ - X 2 ) cos o: + (y^ - y^) sin a. ]j (5) 

y ' sin (of - i3) - sin (a - 0) + cos i3 [ (x^ - X 2 ) cos o? + (y 2 - y^) sin a ]| (6) 

These therefore are the operations^ which giust be performed by the computer. It 
should be noted that when (a - ^) = 0° or 180^ (on the line joining the beacon trans- 
mitters and on the extensions of this line) the error voltages will be zero. There 
is therefore a region of low accuracy adjacent to the base line and its extensions. 
Furthermore, if the aircraft crosses the base line, sin (a - /3) will change sign. If 
the error voltages are to maintain the correct direction of drive, both driving motors 
must be reversed at this point. 





Cb) 


Fig. 18-03 


BENDIX 


18.05 










Il5v. 400~ 


Fig. 18-04 Computing circuits 




18.06 


BENDIX 


Referring to Figure 18-03 (a), the differential drive shown makes the angle 
(o? - 0 ) available as a physical rotation. The switchS, which is closed when 0 < (o? - 
<180^ and open when 180^< (a - )3) <360^, performs the required reversal of the 
motor drive as indicated in Figure 18-03(b), where AC motors are used and the 
sense of the stator fields is reversed by the reversing switch R. 

Referring now to Figure 18-04, AC voltages at 400 cps frequency are taken 
from the secondaries of the eight transformers at the right and are applied to eight 
potentiometers - Pg. The x^, y^ and yg coordinates (constants for the two 

beacons and particular chart used) are preset into P^, P 2 , P 3 , P 4 , P 5 , and P^ as 

indicated. It is suggested that this operation might be ganged with the tuning con- 
trols of the two automatic direction-finders, so that push-button station selectors 
could be used to cover the area within which this type of navigational coverage is 
provided. Pg and Pg are the "position" potentiometers parallel to the x and y axes 
on the plotting board. 

The outputs of P^ and P 2 are placed in series with the stator of A^^, which is 
a goniometer so constructed that the output from its rotor is proportional to the 
cosine of the angle through which the rotor has been turned. Since the rotor is dri- 
ven from then? angular data shaft, the output from it will be a 400-cycle AC voltage 
whose magnitude is proportional to (xj^- X 2 ) cos a. 

In a similar way, Pg, P 4 and A 2 yield an AC voltage whose magnitude is 
proportional to (yg - y^) sin of, the A^ and Ag rotors being at right angles and both 
driven from the a angular data shaft. 

Thesetwo voltages are combined in series and applied to the 6 V 6 torque am- 
plifier. The rotor of Ar^ therefore receives a current proportional to (xj - X 2 ) cos a 
+ (72 - Vl) sin a. 

Ag, A 4 , Ag and Ag are all driven with their stators attached to the a angular 
data shaft and their rotors to the P shaft. Each of them therefore multiplies its 
input voltage by a factor sin (a - /3). The outputs of A 3 and A 4 are placed in series 

and applied to one stator coil of Ary. The rotor of A is driven by the 0 angular data 

shaft. The current in the x output circuit is therefore proportional to 
x' sin(Q? - /S) - ^xi sin (a - 0) + sin/3 [ (xi - X 2 ) coso? + (y 2 - yi) sinQf]| 
which is of the required error form. 

The second stator coil of Arj is at right- angles to the first, and therefore 
multiplies the rotor input by cos /3. The y output is therefore proportional to 
y ' sin (a - /3) - |yj sin (o? - /3) + cos /3 [ (xj - X 2 ) cos a + (y 2 - yi) sina] J 
which is also of the required form. 

Thus if the cursor is not at the correct position, it will be driven there by two 
error voltages (or currents) whose magnitudes decrease as the correct position is 
approached. Anti- hunt features are not included: it is presumed that they will not 
be required. 


CAA VHF Omnidirectional Beacon 


19.01 


Type of system 

Azimuth (radial). 

Useful range 

50 miles at 1000 feet. Coverage area - 50-mile circle around station. 
Accuracy 

(a) Ideal or best theoretical ± 2.8°. 

(b) Actual + 5° (may be improved). 

(c) 180® ambiguity easily resolved. 

Type of presentation 

Right- left zero- center meter and azimuth selector. Neon light indicates 
180® error. 

Operating skill required 

(a) At ground installations: May operate unattended. VHF transmitter, side- 
frequency generator and control equipment to be serviced. 

(b) In navigated craft: Very little skill required. 

(c) Time to obtain a fix: 20 seconds. 

Equipment (Complexity and Weight) 

(a) At ground or control point: T ransmitter and antenna system , fairly complex. 
Weight not in excess of 1000 to 2000 pounds. 

(b) In craft: Receiver employs standard VHF practices. Converter fairly com- 
plex. Weight about 25 pounds. 

Frequency 

TJB mcps. 

Wavelength 

2.4 meters. 

Bandwidth 

About 24 kcps. 

Present status 

Experimental. 

Description of system 

This system is based upon the use of a rotating horizontal- antenna directivity 
pattern. This pattern which is a limacon is produced by an antenna array consist- 
ing of four elements mounted at the corners of a square and a fifth element located 
at the center of the square. The center element is fed with 125-mcps energy which 
is amplitude modulated with a 10- kcps frequency. The 10-kcps frequency is frequency- 
modulated by 60 cps from the power line. The 125-mcps energy supplied to the cen- 
ter element can also be voice-modulated for communication purposes. The other 
four elements are fed with energy in the following manner. Diagonally opposed 
pairs of elements are connected to a common feed point but with different lengths 
of transmission line so that one element is 180® out of phase with the diagonally 
opposite element. The second diagonally opposite pair are phased in the same way. 
Each diagonally opposed pair is fed from a side-frequency generator. The modula- 
tion envelope of one pair of elements is 1/ 4 period different in phase from the mod- 
ulation envelope of the other pair of elements. The side frequencies produced are 
(125 mcps - 60 cps) and (125 mcps + 60 cps). No 125-mcps carrier voltage is pro- 


19.02 


CAA VHF Omnidirectional Beacon 


North 




Fig. 19-01 Principle of operation of omnidirectional beacon 


CAA VHF Omnidirectional Beacon 


19.03 


duced by the side-frequency generators. These side-frequency generators may be 
either balanced modulators or a rotating-capacitor modulator. At any given point 
in space one will receive a 60-cps- modulated 125-mcps signal. The absolute time 
phase of the 60-cps modulation depends upon the azimuth angle with respect to the 
beacon of the receiving point. As one moves about the beacon in azimuth the phase 
of the 60-cps modulation changes. It is thus possible to determine azimuth if some 
phase reference voltage is available. This reference phase is provided by the 60- 
cps frequency modulation of the 10-kcps sub- carrier. 

Figure 19-01 illustrates the operation of the system. It is here assumed 
that the amplitude of the reference voltage passes through a maximum at the same 
instant that the maximum of the rotating pattern passes through North. At the 
point X the variable- phase voltage passes through a maximum 90^ later than the 
reference-phase voltage. This indicates that the point X is at an azimuth angle of 
90® from the beacon. Thus azimuth angles are presented as a phase angle between 
the variable-phase voltage and the reference-phase voltage. 

Two radiating systems have been tried. One consists of five vertical half- 
wave dipoles placed over a circular metal counterpoise. The other radiating sys- 
tem consists of five "Alford loops "^placed above a circular counterpoise. These 
loops are placed in a horizontal plane to give horizontal polarization. It was found 
that reflections from trees, telephone poles and so forth interfered with the accuracy 
of the radiated pattern when the vertical dipoles were used. This distortion of the 
radiation pattern was minimized by using horizontal polarization. The only prac- 
tical omnidirectional elements that can produce horizontal polarization and can be 
closely spaced are Alford loops. Surrounding this array of loops is a vertical 
polarization filter consisting of a cylindrical cage of vertical wires. 



Fig. 19-02 Block diagram of system using electronic side-frequency generators 


*See Figure 19-08 (a) 








19.04 


CAA VHF Omnidirectional Beacon 


Figure 19-02 is a block diagram of the beacon transmitter using electronic 
side- frequency generators. A 125-mcps exciter supplies energy to the two side- 
frequency generators and to the carrier modulator. The modulating voltage for 
the two side-frequency generators is supplied from the 60-cps power line. The 60 
cps supplied to the one side-frequency generator is shifted 90^ with respect to that 
supplied to the other side-frequency generator. The modulation envelopes of these 
two side-frequency generators are thus 90® out of phase. The carrier modulator 
is supplied with a 10-kcps signal which is frequency-modulated at 60 cps to a devia- 
tionratioof 8. In other words, the frequency swings between 9520 to 10480 at a 60-cps 
rate. It was found that frequency modulation of the 10-kcps sub-carrier gave better 
results and less cross modulation than when amplitude modulation of this sub-car- 
rier was used. A speech amplifier and microphone are used so that the center ele- 
ment can also be voice-modulated. 


To one diagonally 
opposed pair 
of elements 



eocps 


Fig. 19-03 Electronic side-frequency generator 


Figure 19-03 is a simple diagram of one of the side-frequency generators. 
It is a balanced modulator using parallel input of the 125-mcps to the two tubes. 
The plates are connected to the output circuit in push-pull and the 60 cps is applied 
to the two plates and screens in push-pull. 

Figure 19-04 is a diagram of the 10-kcps sub- carrier generator. It is a 
delay-line or phase-shift type of oscillator using a 6SG7 amplifier followed by a 
6J5 cathode follower. The output of the cathode follower is fed back to the input of 
the 6SG7 amplifier through a four-section delay line. One of the shunt elements of 
the delay line is made electronically variable by being connected to the two 6J5s 
connected in series. The effective resistance of the 6j5s is varied by the 60-cps 
input. 


Figure 19-05 is a block diagram of the beacon transmitter using a rotating- 
capacitor side-frequency generator. Both the carrier modulation and the side-fre- 
quency modulation are done at the final output level. The rotating-capacitor side- 
frequency generator is driven by a synchronous 60-cps motor running at 3600 rpm. 


CAA VHF Omnidirectional Beacon 


19.05 



Fig. 19-04 Frequency- modulated oscillator 


This motor also drives a two-pole AC generator which supplies the 60-cps reference 
frequency to the 10-kcps FM oscillator. The carrier modulator is a X/4 line type. 

Figure 19-06 illustrates the principle of the rotating- capacitor side-frequency 
generator. It has a rotor consisting of two electrodes supplied with energy from 
the transmitter and two stators mounted at right angles to each other. The rotor 
electrodes are half- cylinders with the ends tapered off. When rolled out flat the 
shape of each side is a double half- sine- wave. Each stator consists of two half- 
cylindrical shells . The two sets of shells are mounted so that the slits are at right 
angles . 


Figure 19-07 is a developed (rolled- out-flat) diagram of the side-frequency 
generator. 

Figure 19-08 (a) shows the design of the Alford loops used. The current 
distribution on two sides is also shown. 

Figure 19-08 (b) shows the top view of the array of five loops. 

Figure 19-08 (c) is a side view of the array of loops. 




19.06 


CAA VHF Omnidirectional Beacon 


carrier Modulator 



^ Fig. 19-05 Block diagram of system using rotating- capacitor side-frequency 

generator 


SHELLS A SHELLS B 



Fig. 19-06 Rotating- capacitor side-frequency generator 


CAA VHF Omnidirectional Beacon 


19.07 


To One D'ngonally Opposed 
Pair of Elements 



To Other Diagonally Opposed 
Pair of Elements 


Fig. 19-07 Developed diagram of rotating- capacitor side-frequency generator 


Figure 19-09 is a block diagram of the converter used in the receiving in- 
stallation. A 125-mcps superheterodyne receiver is used to receive the transmis- 
sions. The converter consists essentially of two channels, a phase shifter and a 
phase comparator. The upper channel contains a high- pass filter and amplifier to 
select and amplify the 10-kcps sub- carrier. This frequency-modulated sub- carrier 
is then applied to a discriminator (shown in Figure 19-10) which recovers the 60- 
cps phase- reference voltage. This 60-cps voltage is applied to a phase splitter and 
then to the two stators of a goniometer-type phase shifter. The lower channel con- 
tains a low-pass filter and amplifier to select and amplify the 60-cps variable- phase 
voltage. The output of the phase shifter and of the lower channel are applied to a 


Current Distribution 



TOP view 
Of Array 



side view 
Of Array 


id) 


ib) 


iC) 


Fig. 19-08 Alford loop and loop array 


19.08 


CAA VHF Omnidirectional Beacon 


phase- comparator circuit. This phase- comparator circuit (shown in Figure 19-11) is 
a wattmeter circuit. The zero- center meter will indicate zero current when the 
voltages are 90O out of phase. This allows a 180^ ambiguity, so an ambiguity-re- 
solving circuit is used. The output from the phase shifter is shifted 445® in one 
phase shifter and the output of the variable-phase channel is shifted -45® in another 
phase shifter. These two voltages are applied to a mixer amplifier and then to a 
neon lamp. When the phase shifter is adjusted to give a zero indication on the left- 
right meter the voltages applied to the mixer will be either in phase or 180® out of 
phase. The correct setting is that which gives in-phase voltages and therefore 
lights the neon lamp. Thus the lamp serves to indicate ambiguity, the beacon code, 
and proper operation of the system. 

Figure 19-12 is a complete circuit diagram of the latest model of azimuth 
converter. 



INDICATOR 


Fig. 19-09 Block diagram of azimuth converter 


CAA VHF Omnidirectional Beacon 


19.09 


0.25 kcp s 




Right-Left 
zero center 
Meter 


Fig. 19-11 Phase comparator 




19.10 


CAA VHF Omnidirectional Beacon 



Fig. 19-12 Circuit diagram of azimuth converter 


CAA LF Omnidirectional Beacon 


20.01 


The Civil Aeronautics Authority is constructing an experimental low fre- 
quency omnidirectional beacon. The principle of operation is the same as that of 
theCAAVHF omnidirectional beacon described in Section 19. Five tower antennas 
will be used instead of the Alford loops used in the VHF version. The frequency 
will be somewhere between 200 kcps and 400 kcps. Because of the low frequency, 
the sub-carrier on the center antenna will be 1000 cps rather than the 10,000 cps 
used on the VHF beacon. Since a rotating-capacitor side-frequency generator is 
not practical at this low frequency it is proposed to use a rotating inductive-gonio- 
meter type of side-frequency generator. The azimuth converter used with the VHF 
beacon can also be used with this LF beacon if the discriminator is changed. 



Federal Long-Range Navigation System 21 .01 


Type of system 
Azimuth. 

Useful range 

Day - 1 500 miles 
Night - 1500 miles. 

Accuracy 

Ideal - Calculated accuracy assuming attenuator accuracy of 1 % . 

Accuracy in best di;rection 

1/6® = 5.2 miles at 1000 miles 
Accuracy 10® from direction of least accuracy 
1-1/2® = 39 miles at 1000 miles 
Actual - Not known 

Ambiguities - Unresolvable ambiguity between sectors spaced equi-angular 
from 90® and 270®. Accuracy in 90® and 270® directions is very poon therefore am- 
biguity between closely spaced sectors on either side of 90® or 270® is not serious 
since system is not useful there anyway. 

Presentation 

Visual (meters and control knobs). Knobs must be varied to give specified 
meter indication and then line of position is read from dials. Line of position can 
be obtained in 1/2 to 1 minute. 

Skill 

Ground: Operator to check phase and amplitudes of currents fed to two 
antennas. 

Craft: Intelligent use to avoid blind faith to readings. 

Equipment required 

Ground: 65-kw (max.) CW transmitter, phase-shifting equipment, two high 
and expensive antennas spaced A/ 2 apart. Relatively simple and could be automa- 
tically monitored. 

Craft: Receiver and indicating equipment. Fairly complex. Special charts. 

Radio-frequency spectrum allotments required 

Frequency: 70 kcps to 76 kcps 

Wavelength: 4280 meters to 3950 meters 
Bandwidth: Receiver - 15 cps 

Transmitter - 70 cps. 


Present status 

Proposed. 

This system is based upon the use of a fixed station which transmits energy 
infour different directivity patterns in succession. The signal received at the craft 
will have four amplitudes corresponding to these four directivity patterns. By pro- 
per interpretation of these values, the azimuth from the fixed station may be deter- 
mined. Let us first consider only two of the four radiated patterns as shown in Fig- 
ure 21-01. Pattern X — is obtained when the two antennas A and B are driven in 
phase. Pattern Y— is obtained when two antennas A and B are driven 180® out of 
phase. If the craft were locate^ at the point f along the line ad the X and Y signals 
would be equal. This would be called an equisignal path since the X and Y signals 
would both correspond to the length ad. Let us now consider the craft at the point 


21.02 


Federal Long-Range Navigational System 



Fig. 21-01 Radiated patterns for 0^ and 180° phase of antennas 


eon the line abc. TheY signal will correspond to the length ac and the X signal will 
correspond to the length ab. If the equipment in the craft can measure the ratio of 
the X signal to the Y signal (X/Y = ab/ac) then it has determined the fact that it may 
be on the line abce. It might also lie on the lines ag, ah, or ak. This ambiguity can 
be reduced from 4 possible lines to two possible lines by making use of the other 
two radiated patterns. In the above case the ratio X/Y would be measured by gat- 
ing the Y signal (ac) through a calibrated attenuator and reducing its amplitude to 
equal that of the unattenuated X signal ab. The value of attenuation necessary may 
be read from the attenuator thus giving the line of position on the special chart. 


Federal Long-Range Navigational System 


21.03 



Fig. 21-02 Radiated patterns for 90^ and 270^ phase of antennas 


In the simplified explanation given using only two antenna patterns, a limited 
number of azimuths in four general directions can be determined since the X/Y or 
Y/X ratio cannot be measured accurately if the weaker signal is too weak. In order 
to overcome this fault four antenna patterns are used. Figure 21-02 gives the two 
additional antenna patterns that are used. Pattern M is obtained when the cur- 

rent in antenna B leads the current in antenna A by 90®. Pattern N — is obtained 
when the current in antenna B leads the current in antenna A by 270®. Thus all 
four of the patterns given in Figure 21-01 and Figure 21-02 may be obtained from 
the two antennas by supplying them with currents of equal magnitude and varying 
the phase of one current in four steps of 90® with respect to the other. Ir order to 
be useful in measuring azimuth, some means of synchronizing the ratio-measuring 
circuit in the receiver must be used. In order to accomplish this an omnidirection- 
al signal is transmitted once each cycle of four directivity patterns. 


21 .04 Federal Long-Range Navigational System 



Fig. 21-03 Cycle of system 


Signol R«c«iv«d oo Lin* ok 


Federal Long-Range Navigational System 


21.05 


Figure 21-03 is a diagrammatic presentation of the currents supplied to each antenna 
and of the directivity patterns that result. The first line represents the current 
supplied to antenna A. This current is constant in phase but varies in amplitude 
as shown. The second line shows the current supplied to antenna B. Its amplitude 
and phase vary as shown. The phase angle indicated is taken with respect to the 
current in antenna A. The third line indicates the antenna directivity patterns 
obtained with the indicated phases. The fourth line indicates the signal that would 
be received along the line ax. The synchronizing signal S is received equally well 
in all directions. This cycle has a period of approximately 1 second. 

The upper part of Figure 21-04 is a rectangular plot of the four radiation 
patterns of the ground antenna system. The bottom part of this figure is a plot of 
the X/Y, Y/X, M/N, and N/M ratios. From this it can be seen that a ratio less 
than 0.4 need never be used. Between the two plots the useful ratios and the sector 
symbol assigned to them are given. It will be noticed that there are two sectors in 
which the same X/Y ratios occur. This ambiguity can be resolved by use of the M/N 
or N/M ratios since they will not be the same for these two ambiguous sectors. The 
ratios used for sector resolution or check are shown above the sector symbol. It 
will be noted that sectors equi-angular distant from 90® and 270® are ambiguous 
and that this ambiguity cannot be resolved. 

Figure 21-05 is a block diagram of the receiver- indicator system. Only the 
IF amplifier of the receiver is indicated. An auxiliary detector is fed from the out- 
put of the IF unit. The output of this detector corresponds to the entire cycle of 
signal strengths. This output is fed to the synchronizing-signal selector which se- 
lects only the synchronizing signal. This signal (pulse) is used to synchronize a 
stable sine-wave oscillator. This oscillator serves as a time base to operate the 
gating circuit. The output of this sine-wave generator is fed to a phase-shifting 
circuit. The output of this circuit consists of two sine waves that are roughly 90® 
out of phase (the actual phase relation proposed is 113°). One of these sine waves 
is used to control a gating-pulse generator. These gating pulses gate an IF ampli- 
fier stage and therefore pass only part of the X and Y signals, or the M and N sig- 
nals. The other sine wave is fed to a half-wave rectifier. The output of this recti- 
fier energizes a nonpolarized relay. The rectifier can be switched to pass either 
the positive or the negative halves of the sine wave supplied to it. As a specific 
example, let us assume that the sine wave of the correct phase to gate the X and Y 
signals to the detector is used. The other sine wave is half-wave rectified and the 
pulses operate the relay. The relay can thus be pre-set, by the rectified pulses of 
current, to pass the X signal through the attenuator and released to pass the Y sig- 
nal direct. The actual duration and timing of the X and Y signals is not determined 
by the relay timing but by the gating pulse generator. By reversing the direction 
of the relay rectifier the Y signal may be passed through the attenuator and the X 
signal passed directly. This permits either the Y/X ratio or the X/Y ratio to be 
measured. By reversing the roles played by the two out-of-phase sine waves the 
M signal and N signal may be gated to the detector and the relay rectifier switch 
permits one to select either the M/N or the N/M ratio. The attenuated and unat- 
tenuated signals are applied to a differential zero-center meter through two revers- 
ing switches and a damping filter. Since this system may be used for homing, one 
reversing switch is used to reverse the sense of the meter indication depending upon 
whether the craft is moving toward or away from the fixed station. The second re- 
versing switch is used to give the correct sense of indication when the ambiguity 
check is made. This second reversing switch, the relay rectifier reversing switch, 
and the phase distributer switch are all ganged together and called the sector selec- 
tor switch. This switch has eight positions, two for each of the following ratio 


AMPLITUDE RATIO AMPLITUDE 


21.06 


Federal Long-Range Navigational System 



21-04 Radiated patterns and ratios used for measurement 




Federal Long-Range Navigational System 21.07 




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21.08 


Federal Long-Range Navigational System 


measurements:X/Y, Y/X, M/N, N/M. For a given value of any one of the four ra- 
tios given there exist four radial lines of position. By checking the value of the 
ratio obtained from the other pair of patterns this ambiguitv can be reduced to two 
lines of position equi- angular distant from the 90^ - 270^ axis. By referring to 
Figure 21-04 it can be seen that a given value of Y/X ratio can be obtained in sec- 
tors C and F. The N/M ratio in sector C is less than one and is greater than one 
in sector F thus permitting the ambiguity between sectors C and F to be resolved 
by a check on the N/M ratio. On the indicating panel a push button is provided to 
do the necessary switching to make this check. This irreducible ambiguity is not 
too serious since the system does not give accurate lines of position close to this 
axis anyway. It is suggested that this ambiguity could be reduced by taking bear- 
ings from other stations or by use of a DF system on the craft. 


The calculated accuracy assuming an attenuator accuracy of 1 % is: 


Pattern 

Azimuth 

Angular error 

Lateral err 
1500 miles 

M-N 

0° (180°) 

1/5° 

5.2 

M-N or X-Y 

ISO (3450^ 1650^ jggO) 

1/3° 

8.7 

X-Y 

30° (330°, 150° 210°) 

50° (310°, 130°, 230°) 

1/4° 

6.5 

M-N or X-Y 

1/2° 

13 

M-N 

700 (290° 110° 250°j 

5/8° 

16 

M-N 

800 ( 280°, 100°, 260°) 

1-1/2° 

39 

M-N 

90° (270°) 

6° 

157 


Some errors might be caused by the fact that the ratio of the field strengths 
is not measured instantaneously but the two fields are sampled in succession. The 
presence of noise or rapid fading would require a very long time-constant in the 
meter damping filter to average out such effects . 


Bibliography 


Identification Classification 


Title 


Issued by 


Proposal No. Confidential 
235 


Universal Communications, Federal Tele- 

Airport Control, Traffic phone and Radio 

Control, and Aerial Naviga- Corporation 

tion System (Part III - Aer- 
ial Navigation) 


Airborne Radar 


22.01 


There are a number of airborne radar sets which are designed to perform 
one or several different functions such as: search, general navigation (by beacons, 
landscape, etc.), bomb release, identification, interception, early warning, gun- 
laying, collision prevention, measurement of altitude, etc. Brief descriptions of the 
various pieces of radar equipment which are available to perform some of the 
above functions may be found in Section 1 of the U.S. Radar Survey. H2X (AN/APS-15 
or AN/APQ-13) equipment is among the most useful airborne radars for purposes 
of general navigation, and has been in operational use for several years. AN/AM-15 
is typical of Hffi equipment. The following pages of this section contain a descrip- 
tion of the AN/APS-15 set and the NOSMO attachment to it. The Micro-H attach- 
mentfor H2X is described separately in Section 5. AN/APS-15 is X-band airborne 
radar equipment with PPI presentation and facilities for making accurate range 
measurements to range- coded beacons. 

Description of Radar Set AN/APS-15 (H2X) 


Type of System 

Combination of range and azimuth. 

Useful Range 

0-90 miles for search operation, 0-250 miles for beacon operation (radar 
line- of- sight), 1000-36,000 feet for precise altitude measurement. Battleships can 
be detected at a maximum reliable range of 60 miles, destroyers at 45 miles, sur- 
faced submarines at 10 miles, and groimd targets such as cities at a maximum range 
of 30-60 miles. 

Accuracy and Precision 

(a) Range accuracy: For beacon interrogation, + 200 yards; for high altitude 
bombing + 200 yards; for normal search operation, + 500 yards on 5-mile sweep, 

+ 5,000-10,000 yards on 90-mile sweep; for high altitude search, ± 200 yards on 
computer, ± 500 yards on 5 -mile sweep, + 2000 yards on 20 -mile sweep. 

(b) Azimuth- angle accuracy, ± 3^; azimuth- angle accuracy with azimuth 
stabilization, + 6^. 

Presentation of Data 

Visual on PPI. 

Operating Skills Required 

One skilled radar- operator and possibly also a navigator in the aircraft. 
Ground beacons are unattended. Time for a navigational fix with beacons: less than 
a minute. 

Equipment Required 

Weight of H2X aircraft equipment is about 370 pounds. An X-band beacon 
such as AN/CPN-6 or equivalent is suitable as ground beacon equipment for use with 
H2X. 

Radio- Frequency Spectrum Allotments Required 

Aircraft Radar, f = 9335 to 9415 mcps X = 3.2 cm. B.W. = 2.5 mcps. 

Groimd Beacons, f = 9310 mcps 

Present Status of Development 

Fully developed, in production, and in operational use. 

Brief Non-Technical Description of H2X Equipment 

Although the primary use of H2X equipment is to locate land objectives and 
to time the release of bombs for area bombing, it is also an extremely useful aid 


22.02 


Airborne Radar 


for general navigation. H2X equipment provides means for: 

(1) Blmd navigation and homing through beacon reception. 

(2) Normal radar search operation with PPI presentation. 

(3) Control of high- altitude bombing. 

(4) Determination of altitude. 

(5) Instantaneous determination of true or relative bearing. 

(6) Determination of drift angle. 

The essential components of the set are shown in the block diagram of Fig- 
ure 22-01. 



TARGET 


Fig. 22-01 Simplified block diagram for H2X radar set 


The PPI gives a map- like presenta- 
tion of a large circular area (or sector 
thereof) of the earth' s surface, and super- 
imposed upon this may be indications of 
other aircraft in the vicinity. During navi- 
gation by beacons, only beacon response 
signals and an adjustable rang^ marker 
appear on the PPI screen. 

The map- like character of the PPI 
presentation makes it extreme y useful as 
a general navigational aid especially near 
coastal regions where characteristic ir- 
regularities of coastline are easily recog- 
nizable. The top of the PPI screen may be 
chosen to represent either true north or 
the heading of the aircraft. 



The altitude of the aircraft is ob- ^ 

tainedfrom the time required for a pulse Fig. 22-02 Example of beacbn 

signal to travel from the aircraft to the code signal 

ground and back. Two signals must be 

aligned visually on a separate cathode -ray- tube indicator (A-scan) with the aid of a 
variabletime-delay circuit the adjustment of which controls the position of a direct- 
reading altitude- indicator. 






Airborne Radar 


22.03 


Radar responder beacons (racons) may be interrogated from the aircraft, 
and range-coded response signals observed on the PPI screen as illustrated by 
Figure 22-02. The beacon response signals are coded for the purpose of beacon 
identification. The range to any beacon may be obtained from dial indications after 
setting an adjustable slant- range marker circle for coincidence with the first (in- 
ner) beacon response on the PPI. From the altitude and slant range, and ground 
range may be easily determined. (See Figure 22-03). Ground range determinations on 
two beacons enable the navigator to deter- 
mine a fix (See Figure 22-04). 


The PPI may be set so that the top 
of its screen represents the heading of the 
aircraft. It is then a simple matter for the 
pilot to direct the craft so as to "home" on 
a beacon. 



Fig. 22-03 Range and altitude 
relationships 


The stationary circumference of the PPI screen is marked off in degrees 
with zero at the top. A fine ruled line on a transparent rotatable cover may be set 
to any desired azimuth. When the flux-gate compass and associated azimuth-stab- 
ilization equipment is used to keep the true- 
north direction at the top (0° position) of the 
PPI screen, the adjustable azimuth line on 
the transparent cover may be conveniently 
used to obtain the true bearing of any tar- 
get appearing on the screen. A bright radial 
line on the PPI is available if desired to 
indicate the heading of* the aircraft. If the 
azimuth stabilization equipment is not used, 
then the top of the PPI image represents 
the heading of the aircraft; and the adjust- 
able azimuth line may then be used to deter- 
mine the relative bearing of any target with 
respect to the heading of the aircraft. The 
use of the adjustable azimuth line to deter- 
mine drift angle will be discussed later. 

Although this survey is not primar- 
ily concerned with bombing- run procedure, in the present instance an outline of the 
high- altitude bombing operation provides a convenient illustration of techniques 
which are also applicable to general navigational problems . In this operation, a pre- 
liminary determination of wind velocity must be made early in the mission before 
the start of the actual bombing run. This may be accomplished by determining the 
air velocity and ground velocity during a short straight flight. The air velocity is 
obtainedfrom the direction of flight (PPI indicator) and an air-speed indicator dial, 
the reading of which must be corrected for altitude and temperature. The ground 
velocity may be most easily determined with the aid of ground beacons. Two suc- 
cessive navigational "fixes", taken a short time apart, will give the ground velocity 
(ground speed and course bearing). A vector diagram such as that shown in Figure 
22-05 enables the navigator to determine the wind velocity. 



Fig. 22-04 Navigational fix 
obtained from range 
measurements to two 
beacons 


As soon as the target is located (within 50 miles of the aircraft) the pilot 
heads the aircraft toward the target and the navigator calculates as quickly as pos- 
sible what the air speed and heading should be in order that the plane’ s course pass 
directly over the target. This trigonometric calculation is simply a second applica- 


22.04 


Airborne Radar 


tion of a vector diagram similar to that of 
Figure 22-05, except that now the desired 
ground velocity toward the target is known; 
and from the predetermined wind speed 
tlje navigator can calculate the required 
air speed and heading of the aircraft. 

Temperature and altitude corrections must 
be applied to the true air speed in order 
to obtain the indicated air speed to which 
the pilot must hold the aircraft. 

If the center line of the sector scan 
corresponds to the heading of the aircraft 
then the top of the PPI screen indicates 
the craft heading; and the adjustable azi- 
muth line if set at the proper drift angle 
indicates the aircraft* s course over the 
PPI map. All objects on the PPI screen 
tend to drift parallel to the adjustable azi- 
muth line underneath which the target 
shouldappear. However, during the bomb- Fig. 22-05 Determination of 

ing run it is more convenient to have the wind velocity 

top of the PPI screen correspond to the 

aircraft course through the target as shown in Figure 22-06. An azimuth adjustment 
is therefore provided so that the center of the sector-scan may be displaced from the 
heading of the aircraft. 

For a given type of bomb, corresponding altitude and slant ranges have been 
determined for several values of ground speed. This information is presented graph- 
ically upon a drum chart, the vertical and horizontal scales of which are altitude and 
slant range. Adjustment of a slant- range cross hair for coincidence with an inter- 
section of an altitude cross hair with the chosen value of aircraft ground speed, auto- 
matically places the adjustable slant- range marker-circle on the PPI at the proper 
slant range required for bomb release. The bombs are released at the instant 
that the target crosses tne slant-range marker- circle. 



Brief Technical Description of the 

AN/APS-15 (H2X) Radar^ 

The type AN/APS-15 radar set 
has been chosen as typical airborne 
equipment. A brief description of 
the type and function of each com- 
ponent is presented first. T hen a number 
of more specialized functional block-dia- 
grams with some waveforms and time 
charts are discussed. A few circuit dia- 
grams of special interest are given; but 
for detailed mechanical and electrical in- 
formation the reader is referred to the 
Handbook of Instructions for Radio Set 
AN/APS-15 (H2X) prepared by the MIT 
Radiation Laboratory. Space does not 
permit a detailed description of each and 
every circuit of the H2X system. More 
space is allotted to the description of the 
timing circuits than to more standard 
components such as IF amplifiers, wave 



Fig. 22-06 Setting of precise 
slant range marker 


Airborne Radar 


22.05 



Fig. 22-07 Functional block diagram of the AN/APS-15 equipment 

guides, etc. For a description of the more or less standard radar- set components 
and special UHF techniques involved in the operation thereof, the reader is referred 
toany of theArmy or Navy radar instruction manuals or to other similar books such 
as "Principles of Radar" by the staff of the MIT Radar School. 

Figure 22-07 is a functional block diagram of the AN/APS-15 equipment. 
Ref erring to Figure 22-07, the timer or control central, the computer, and the range 
unit are the components of greatest interest in a discussion of the fimctional oper- 
ation of H2X equipment. The consideration of these timing circuits is postponed 
until a brief description of the other components of the system has been completed. 

Sweep Circuits 

The "A" scope may be used for a variety of purposes such as checking the 
dividing ratio of a frequency- dividing circuit, checking signal strength and overload 
of the receiver, or other alignment problems; but the operational use of the "A" 
scan is primarily for matching a delayed marker pip with the first ground- return 
signal for the determination of altitude. The sawtooth voltage sweep generator for 
the "A" scope is a conventional RC hard- tube circuit in which the desired sweep is 
generated while a normally conducting tube is cut off by a negative gate from the 
timing oscillator. The same negative gate is used to "imblank" the "A" scope dur- 
ing the sweep. 

The trapezoidal voltage necessary to cause a sawtooth current- wave in the 
magnetic deflection coils of the PPI tube is generated by a circuit very similar to 
that of the "A"-scope sweep- generator. The required trapezoidal voltage is fed 
through slip rings to the rotor of the sweep synchro which rotates in synchronism 
with the antenna system (See Figure 22-08). Two mutually perpendicular sets of 
secondary stator windings each pick up a trapezoidal voltage and apply it through a 
push-pull synchronized clamping- circuit to a correspondingly oriented set of mag- 
netic-deflection coils on the PPI tube. The rotary motion of the synchro rotor is 
relatively slow, so that its change in angle during the short time of any one trape- 
zoidal sweep voltage cycle is negligibly small. Consequently the voltages induced 
in the two mutxially perpendicular sets of secondary stator coils are in time phase 
with one another, but in general differ in magnitude, depending upon the position of 






22.06 


Airborne Radar 



A similar circuit connected to stator II provides for vertical deflection. 
The normally conducting tubes VI and V2 act as a voltage divider to clamp the con- 
trol grid of V5 to a definite potential except during the time of the sweep when a neg- 
ative gate cuts off VI and V2 allowing the tropezoidal signal to be amplified by V5. 
The lower half of the push-pull circuit acts in a similar manner. Horizontal center- 
ing is accomplished by balancing the DC plate currents of V5 and V6. 


the rotor. The magnitudes of the two secondary voltages vary slowly throughout the 
cycle of rotation of the antenna spinner (12 or 24 r.p.m.); the magnitude of one vary- 
ing sinusoidally and that of the other cosinusoidally with respect to the angular posi- 
tion of the rotor. These voltages provide a radial sweep (from the center to the 
outer edge of the screen) the azimuth of which changes in synchronism with the 
direction of the antenna beam. The top of the PPI presentation corresponds to the 
heading of the aircraft unless azimuth stabilization is used, in which case the position 
of the sweep- synchro stator coils relative to the aircraft heading may be oriented 
by means of a flux-gate compass and servo- system (including magnetic deviation 
correction) so that the true north direction appears at the top of the PPI screen. 

RF Components 

The modulator, transmitter and receiver-preamplifier (converter) are locat- 
ed in a pressurized chamber directly above the radome. The input pulse to the 
driver or pulse-forming circuit is shown 
in Figure 22-09 and has a peak amplitude 
of about 120 volts. This voltage is divid- 
ed between an artificial line, a transform- 
er secondary S, and the grid- cathode 
space of the pulse-forming amplifier tube 
shown in Figure 22-10. The transformer 
has three suitably damped windings with 
the primary and secondary windings re- 
generatively connected. The tube is 
normally biased beyond cut-off, but upon 
application of the positive pulse, the 
regenerative action turns on the plate 
current suddenly. The rise of plate 
current is, however, quite linear, result- 



Fig. 22-09 Input pulse to the driver 


Airborne Radar 


22.07 



Fig. 22-10 Transmitter driver 
pulse forming 
circuit 


ing in constant secondary and tertiary 
voltages. The uniform rise of plate cur- 
rent continues until the input pulse returns 
as an in-phase reflection from the end of 
the artificial line, at which instant the grid 
is driven negative and regenerative action 
abruptly stops the flow of plate current. 
A typical rectangular output voltage pulse 
of the tertiary winding is shown in Figure 
22-11. The time width of the output pulse 
of course depends upon the delay introduc- 
ed by the artificial line. Three artificial 
lines are available for generating 0.5, 1, 
and 2 microsecond pulses. 

Application of the positive, rectang- 
ular pulse to the grid of a power-amplifier 
stage, delivers a 14,000 volt negative 
pulse to the cathode of the magnetron 
oscillator, the plate of which is at ground 
potential. 


The short train of SHF electro- 
magnetic waves constituting a signal 
pulse is transmitted from the magnetron to the antenna through the radio- frequency 
system, consisting of the magnetron coupling unit, T-R and R-T boxes, several 
lengths of rectangular waveguide (1" x 1/2”), azimuth- rotating and tilt-rotating 
joints. The antenna consists of the antenna feed system and specially- shaped re- 
flector. A block diagram of the pressurized RF system is given in Figure 22-12. 
The function of the T-R box is to protect the crystal of the sensitive receiving- sys- 
tem from the high-power transmitted pulse. The output circuit of the magnetron 



Fig. 22-11 Driver output pulse 
during beacon 
operation 


when inoperative is prevented by the R-T 
box from absorbing much energy from the 
weak radar echo signals. 

The wave guide terminates in two 
"windows” at the focal point of the reflec- 
tor. The radiation from the windows is 
gathered by the reflector and focused into 
a narrow, fan- like beam of about 3^ angu- 
lar width in the horizontal plane, and about 



Fig. 22-12 Block diagram of the 
RF system 


22.08 


Airborne Radar 



Fig. 22-13 Perspective drawing of antenna beam pattern 


60^ angular width in the vertical plane (assuming the aircraft in level flight). Figure 
22-13 illustrates the illumination of the ground when the ref lector is in its normal 
position with the aircraft in level flight. The small area within curve A receives 
the maximum illumination with very slight variation. This small area is at an angle 
of depression 0 of about lOo from the horizontal (See Figure 22-14). The illumina- 
tion decreases gradually between curves A and B until at curve B it has fallen to one- 
half of its maximum value (3 db down). This type of beam pattern is sometimes re- 
ferred to as a "cosecant squared" type, since over a considerable range of 0 the 
field strength in a vertical plane containing the aircraft and its line of flight is 
directly proportional to cosecant 20. The reflector may be tilted in the vertical 
plane by + 20 degrees from its normal position in order to permit a more favorable 
illumination of a particular target area. The antenna system may be rotated contin- 
uously to provide for a 360° PPI presentation, or it may be wobbled back and forth 
to provide for a 56° sector scan. 

Echo signals may be received only from the direction in which the reflector 
is "looking". Reflected wave trains are collected by the reflector and focused upon 
the windows in the end of the wave guide which act as a receiving system to driy^ 
the wave guide. Such a reflected signal traverses the RF system in the opposite 
direction from that of the transmitted pulse. At the junction leading to the T-R box 
however, the received signals are diverted through the T-R box to the converter of 
the receiving system. 

Flux- gate Compass System 

The heading of the aircraft relative 
to true or magnetic north is determined by 
the gyro flux- gate compass system which 
depends for its operation upon the hori- 
zontal component of the earth' s magnetic 
field. The flux- gate element consists of 
three soft iron cores, each carrying pri- 
mary and secondary windings and arranged 
as the legs of an equilateral triangle as in- 
dicated in Figure 22-15. It is located out 
in one wing of the aircraft, as far as pos- 
sible from any ferrous -metal parts, and is 



Fig. 22-14 Approximate antenna 
radiation pattern for 
constant target contrast 


Airborne Radar 


22.09 



Sweep Selsyn with Adjustable 
Azimuth Slotor Coils 


Rotor Spins with 
Antenno System 


Fig. 22-15 Schematic diagram showing essential elements of azimuth 
stabilization system 


stabilized in a horizontal plane by a vertical- seeking gyroscope, the electric driv- 
ing-motor of which is powered by the 400 cps supply of the aircraft. 

A 2.5 volt sine wave from the 487.5 cps oscillator is impressed upon the 
primary winding of the flux gate. This signal is sufficiently large to saturate the 
cores so that the only significant rate of change of flux occurs twice a cycle when 
the primary current is near zero. The primary current which saturates the cores 
serves to gate the effect of the earth' s magnetic field in such a way that the earth' s 
field can exert its influence only during two short periods in each cycle, hence the 
name "flux gate". If the earth' s magnetic field were non-existent, the three second- 
ary voltages would all be identical, and would consist of alternate positive and nega- 
tive pulses occurring whenever the primary current passed through zero. The 
presence of the earth' s magnetic field or a component thereof parallel to one of the 
cores will cause an increase in the secondary voltage pulse of one polarity and a 
decrease in the size of the pulse of opposite polarity. The presence of alternate 
positive and negative pulses of \mequal magnitude is essentially equivalent to a 
large second- harmonic component in the secondary voltage. The second harmonic 
content of the voltage pulses in the three secondary windings will in general be dif- 
ferent for each of the windings, and will depend upon the direction of the earth' s 
horizontal field relative to the fixed orientation of the flux- gate element in the air- 
craft. If the earth' s magnetic field were non-existent, the equal secondary voltages 
of the flux gate applied to the stator of the autosyn would produce no resultant magne- 
tic field to act on the rotor; but with the earth's field acting on the flux gate, the 
unequal voltages applied to the stator of the autosyn produce an alternating field 
having a definite direction dependent upon the direction of the earth' s field linking 
the flux gate. The rotor of the autosyn picks up a voltage, the 975 cps component 
of which is selected by an amplifier and band pass filter and applied to one phase 
of a two phase induction motor, the other phase being fed by a 975 cps signal from 
a doubler oscillator. The motor turns until the rotor of the autosyn reaches a null 
position in which no voltage is picked up to drive the motor further. The rotor is 
geared to a dial pointer which indicated the heading of the aircraft. Provision is 
made so that the magnetic variation may be set in manually so that the dial pointer 





22.10 


Airborne Radar 


of the master indicator reads the heading of the aircraft relative to true north 
instead of magnetic north. Mechanical screwdriver adjustments are provided every 
15 degrees around the circumference of the dial to correct the pointer reading for 
errors introduced by the distortion of the earth' s field due to magnetic material in 
the aircraft. The calibration adjustments are made at a selected ground site 
after installation of the equipment. 

In order that the top of the PPI screen shall always represent the north 
direction, as the aircraft heading changes, the movement of the sweep- synchro 
stators must follow the rotation of the autosyn rotor. This is accomplished by 
means of another servo link. A permanent magnet rotor of a transmitting magnesyn 
is geared to the pointer of the master indicator. 400 cps power is fed to the satur- 
able core stator windings of the transmitting magnesyn and also to one phase of a two 
phase induction motor M 2 . The voltage distribution in the three stator windings of 
the magnesyn as determined by the position of its rotor is transmitted to the stator 
windings of the torque synchro and sets up an alternating field in a direction corres- 
ponding to the position of the permanent magnet rotor of the transmitting magnesyn. 
The rotor coil of the torque synchro picks up an error voltage which is amplified by 
the torque amplifier and applied to the second phase of the motor M 2 . The motor 
runs, rotating both the sweep synchro stator assembly and the rotor coil of the torque 
synchro until the coil reaches a null position in which no voltage is picked up to 
drive the motor further. 



Fig. 22-16 Block diagram of the receiver showing signal path 


Receiver Components 

A block diagram of the receiver is shown in Figure 22-16. The frequency 
used for the beacon reply signals is slightly different from that of the magnetron 
transmitter, and it is convenient to provide two local oscillators, one for use in 
normal search operation and one for use in beacon navigation. The receiver is 
divided into two main parts as indicated in the figure. The T-R box, converter, and 
first two IF amplifier stages are located near the antenna system in the same pres- 
surized container which houses the transmitting magnetron and pulse-shaping com- 
ponents. The receiver proper is located in the Receiver-Indicator unit in front of 
the radar operator at some distance from the antenna system. The receiver proper 
contains five IF stages, 2nd detector, two video- stages and also a circuit for auto- 
matic frequency-control of the local oscillator. 

RF and local- oscillator signals are fed into the converter or mixer which 
is a section of wave guide terminated by a crystal. Both echo and beacon local-oscil- 
lators employ the 723-A Shepherd-Pierce tube, a velocity modulated tube of the. re- 
flex type. The local oscillator frequency is 30mcps higher than the RF frequency and 









Airborne Radar 


22.11 


is maintained at the correct frequency by an automatic frequency- control circuit 
which feeds back a DC control- voltage to the target electrode of the local oscillator 
tube. 

A typical IF stage is shown in Fig- 
ure 22-17. All IF stages are of the "sin- 
gle-tuned” type with slug tuning of the in- 
ductors in the grid circuit. The input and 
output capacitances of the tubes comprise 
the major part of the circuit capacitance. 

A conventional diode- detector and 
two video- amplifier stages are used, the 
second video- stage being a cathode-follow- 
er, controlling the PPI beam intensity. 

Range- marker pips may be mixed with the 
signals at the input to the first video stage. 

Timing Circuits 

Referring to Figure 22-07 the timer, or control central, is that part of the 
radar set which initiates and controls the timing of pulse transmission, indicator 
sweeps, gates, range markers, etc., that is, it acts as the stop watch of the system. 
The main component of the control central is a master multivibrator or timing 
oscillator which may be either "free running", or synchronized by every 70th or 
320th pulse originating in a crystal oscillator running at 80.86 kcps located in the range 
unit. The repetition rate of the synchronizing pulses is stepped down from that of 
the crystal oscillator by two frequency- dividing circuits of the blocking- oscillator 
type in steps of 10:1 and then either 7:1 or 32:1 depending upon whether the set is 
beingusedfor high- altitude search or beacon navigation. During beacon observation 
the pulse repetition rate must be low in order to allow sufficient time for a beacon 
response to return from a distance which may be as great as 250 nautical miles. 

Three separate gate-signals are taken from, the multivibrator to initiate 
and control certain system functions to be described shortly. The frequency or over- 
all period is constant, but four choices of relative conducting times of the two 
triode- sections of the multivibrator are available for the 5, 20, 50, and 90-mile 
sweeps. The timing circuits of the range unit are each described in turn before 
any functional block diagrams of the system as a whole are described. 

Figure 22-18 is a block diagram of the control central and indicators. A 
few typical voltage waveforms are shown for the 20- mile sweep. Gating voltages 
from the timing oscillator (master multivibrator) simultaneously initiate and con- 
trol the time duration of the "A" scope sweep generator, the "A" scope unblanking 
circuit, and the PPI sweep generator circuits. Due to time delays in the sweep 
synchro, the unblanking of the PPI sweep must be delayed by 12 microseconds in 
order to allow time for the sweep to get started. The delay and pulse forming is 
accomplished with two triodes shown in Figure 22-19. A negative exponential grid- 
waveform from the master multivibrator cuts off one triode which is normally 
conducting. A capacitor connected between itfe plate and ground charges toward 
B+ through an adjustable plate load resistor. The plate of the first triode is direct- 
ly connected to the grid of a second triode which is normally cut off. The grid 
voltage of the second triode rises exponentially and reaches cutoff in about 12 micro- 
seconds. The exponential change of grid voltage is amplified and the plate voltage 
used to gate both the PPI unblanking circuit and the ringing circuit of the range- mark 
generator. The initial rate of change of plate voltage on the second triode is suf- 
ficiently great so that it may be differentiated by R 2 C 2 to form a pulse. The pulse 
is then amplified and coupled through a cathode follower to trigger the modulator. 



Fig. 22-17 Typical intermediate- 

cfonro 


22.12 


Airborne Radar 


•a* scope 

SWEEP 

GENERATOR 

I /I V- tt» 


*A* SCOPE 
UN8LANKING 
l/t »-tll 


’•a* 

SCOPE 



X. 


ALTITUDE MARKER FROM RANGE UNIT. 


•a* scope 
amplifier 

l/e V-tit 


VIDEO 



COUNT 


DISC 


TIMING 

OSCILLATOR 


V- EtA 



TRIGGER 

FROM 

RANGE UNIT 


PEAKER 


PPI 

SWEEP 

GENERATOR 

I/I v-tti 



SELSYN 

DRIVER 



deflection 

AMPLIFIER 



¥- tit 


CLAMPER 

V- 114 


CLAMPER 

V-llS 


JjLjL!L5U- 


U poiiw 


SELSYN 


DEFLECTION 
AMPLIFIER 
V - tit 


deflection 

AMPLIFIER 
V- HO 


CLAMPER 
¥ - tit 

ZE 


CLAMPER 

V-IIT 


DEFLECTION 

amplifier 

¥ - III 


PPI 

UNBLANKINC 
i/t ¥-tlt 



VIDEO SIGNALS 
FROM 
RECEIVEK 


deflection 

COILS 


VMVEFORMS SHOWN 
FOR 20 mile sweep 



DELAY 


pulse 

DELAY 

l/t V- tl« 


RANGE MARK 

■•“GENERATOR 


'f 

AMPLIFIER 

AND 


RANGE 

MARK 

l/t v-llt 



amplifier— ► 

V* ttt 




SHAPER 

V- tit 


AMPLIFIER 
l/t fill 


TO RECEIVEf 


trigger 

AMPLIFIER 
AND CATHODE 

follower 

¥ T t|Y 


AIOOULATOR 


TRIGGER 


Tm 


tlFlCO AtOuT 10 TiMtt 


Fig. 22-18 Block diagram of control central and indicators 


the operation of which has already been covered. As described later, the modulator 
may also be triggered off by a pulse from the range unit whenever the sweeps are 
delayed by the altitude delay circuit, or by both the altitude and beacon delay cir- 
cuits . 


Airborne Radar 


22.13 



The range- mark generator shown in Figure 22-20 employs a conventional 
ringing circuit. During the quiescent period near the end of each cycle when the 
grid of the first triode is at ground potential, it conducts a steady plate current 



Output 


from t.F.F TO 
BWsJ&C MAEJC. 
AMPUPIEf^ 


Fig. 22-20 Range mark generator and amplifier 


which passes through the inductor of any one of four available tuned cathode cir- 
cuits. No oscillations can take plate at this time because the tuned circuits are 
highly damped by the very low output- impedance of the first tube. Upon the applica- 
tion of a negative gate (from the pulse-delay circuit previously described) to the 
grid of the first triode section of the range- mark generator, the plate current is 
cut off; and the energy stored in the inductance of the tuned circuit initiates a train 
of damped oscillations. The damped oscillations appearing at the cathode of the 
first triode (see Figure 22-21), are amplified, clipped, and differentiated in the 
remaining triode sections of the range- mark generator, the differentiation occuring 
in the inductive plate- circuit of the last triode section. The range- mark pips are 
amplified in a single stage and mixed with the echo signals in the video amplifier 
which feeds the intensifier grid of the PPI tube. 

The negative gate from the 12- microsecond pulse-delay tube is used to un- 
blank the PPI during the sweep period. The unblanking circuit and PPI operating 
circuits are shown in Figure 22-22. 

Figure 22-21 shows the time coincidence of some of the more important 


22.14 


Airborne Radar 


0 


"T 

10 


-| 1 1 1 1 1 I I r* Time in microseconds 

20 30 40 50 60 70 80 90 100 



i 2 >j sec- 


PPl sweep triggered 
^ ^PPI sweep gets starts 


K — PPi unblanked during this time intervol 



“W' sweep 


< "a" sweep unbtanked during this interval 



Fig. 22-21 Time diagram for five mile sweep 


Airborne Radar 


22.15 



Fig. 22-22 PPI operating circuits 


events occurring in normal search operation. The five- mile sweep was chosen for 
convenience in plotting. 5, 20, 50, and 90-mile sweeps are available. Figure 22-23 
is a "stop watch" diagram showing for the 90- mile sweep the sequence of events 
occurring in normal search operation. 

With the aid of the ordinary equally- spaced range- marks on the PPI, measure- 
ment of range can be interpolated with a precision of + 500 yards. The precision 
of range measurement is increased to + 200 yards by the use of a precise range- 
marker of adjustable range. The additional timing- circuits necessary to control 
the precise range marker are contained primarily in the range unit, but partly in 
the computer and control unit. 

The period of an 80.86 kcps signal corresponds to the time required for an 
electromagnetic wave to travel one nautical mile and return. An 80.86 kcps triode 
crystal oscillator (V-1 of Figure 22-24) is the source of all timing pulses in the 
range unit. The operation of the crystal oscillator is class C, and the plate current 
pulse serves to trigger the blocking- oscillator pip- generator. The phasing of the 
pulse is such that the oscillator plate- cur rent- pulse causes the grid of the blocking 
oscillator to swing in the positive direction and the plate to swing in the negative 
direction. The natural period of the blocking oscillator is of course slightly longer 
than that of the crystal oscillator so that the blocking oscillator grid potential will 
not have risen to cut-off potential by the time the triggering pulse arrives. 


22.16 


Airborne Radar 


90 MILE SWEEP IS SHOWN 
ALL TIMES IN MICROSECONDS 



Fig. 22-23 "Stop-watch" diagram of operating sequences in normal search 

operation 



Trigger to 
phantastroT''. 


Modulator 

trigger 


Fig. 22-24 Crystal controlled timing circuits 



Airborne Radar 


22.17 


The negative output pulse from the pip generator serves to trigger the 10:1 
frequency- dividing circuit which is also of the blocking- oscillator type. The output 
of the 10:1 divider feeds 10- mile pips to a gated amplifier yet to be described and 
also triggers the final frequency- dividing circuit which divides in either a 7:1 or 
32:1 ratio depending upon whether the bottom resistor in the grid circuit is shorted 
or not. The output of the final frequency- divider is coupled through a cathode fol- 
lower to a cable leading to the modulator in the transmitter unit. A voltage divider 
from B+ to ground through a portion of the cathode circuit provides sufficient bias 
to keep the cathode-follower tube cut off until the pulse from the frequency- divider 
circuit reaches a certain level. This arrangement eliminates the first, slowly rising 
portion of the applied signal and results in an output pulse with a much steeper wave- 
front. A negative pulse to trigger the phantastrons is taken from the plate of the 
combination amplifier and cathode follower stage. 

The Phantastron Delay Circuits 

A phantastron delay circuit is a voltage- controlled, medium precision, 
aperiodic delay- system designed for range measurement. The control voltage is 
effective over a 200 volt range, and the time delay or "gate width” introduced by the 
phantastron is accurate to within 0.1 microsecond corresponding to the time requir- 
ed for an electromagnetic wave to travel about 30 yards . T his is equivalent to 1 5 yards 
of measured range (allowing for go- and- return). The accuracy of range measure- 
ment is about ±15 yards ± 0.1 percent of the range measured as far as the phan- 
tastron circuit alone is concerned; but the full capabilities of the circuit are not 
realized in the H2X system in which the operational accuracy of range measure- 
ment is considerably less than the above figure because the signal matching for range 
measurement is done on a rather limited PPI sweep. The output of the phantastron 
is free from jitter to within ± 5 yards for delays less than 200,000 yards, the delay 
time being practically independent of the repetition frequency as long as the delay 
does not exceed 90 percent of the period. 

The advantage of the phantastron over a multivibrator is that the delay time 
is almost independent of the supply voltage, and depends instead upon a ratio of 
voltages determined by the setting of a potentiometer connected across the supply 



Fig. 22-25 Phantastron delay circuit 


22.18 


Airborne Radar 


+250V 



Fig. 22-26 Pulse amplifier used between a phantastron delay circuit and a circuit 

to be triggered by it 

voltage. The delay time also depends upon one temperature-controlled RC time 
constant. However, the phantastron does not turn off as sharply as a multivibrator 
and so the output must be amplified in order to obtain an output signal useful for 
triggering. 

A phantastron delay- circuit is diagrammed in Figure 22-25. It may be trig- 
gered in several different ways, but the input triggering- pulse is usually applied to 
the number three grid as a positive pulse. The output voltage, taken from the 
cathode of the 6SA7 phantastron, is used to control a pulse amplifier of the type 
shown in Figure 22-26. The amount of time delay introduced by the phantastron is 
determined by the setting of the control- voltage tap on the potentiometer at the left 
of Figure 22-25. The triode cathode-follower on the right is used to provide a path 
for the quick charge of condenser C at the end of the delay cycle, thus shortening 
the recovery time so that the circuit may accept another triggering impulse. Since 
the gain of the cathode follower is essentially unity, the phantastron circuit is equi- 
valent to the simplified diagram of Figure 22-27 which is convenient for visualizing 
the circuit operation. 




Fig. 22-27 Simplified phantastron circuit 


Airborne Radar 


22.19 


For purposes of explanation, the operation of the circuit may be convenient- 
ly divided into six stages. A few typical voltage wave-forms are given in Figure 
22-28. 


Stage VI is the quiescent stage in which the phantastron circuit is waiting 
for a triggering impulse to start a cycle of operations. In this stage there is consi- 
derable screen current (mainly from the inner screen G 2 ) and very little plate cur- 
rent. During this stage the plate potential of the phantastron (6SA7) is determined 



DENOTES TRIOOER APPLIED SIMULTANEOUSLY TO PLATE AND FIRST GRID. 

Fig. 22-28 Waveforms of phantastron delay voltages with respect to ground 


quiescent stage 


22.20 


Airborne Radar 


by the setting of the control voltage potentiometer tap. Grid current flows through 
Rg and keeps the potential of approximately at cathode potential which is about 
40 volts above ground for the case illustrated in Figure 22-28. 

Upon the application of a positive triggering pulse to G 3 or a negative trig- 
gering pulse to the plate or to the plate and Gi, the current flowing to the 
screen grids (G 2 , G^) is suddenly shifted to the plate circuit. The plate potential 
drops by some 30 or 40 volts during stage I due mainly to the increase of plate 
current through resistor Rj^, and the left- hand diode of Figure 22-25 stops conduct- 
ing. The voltage on capacitor C does not change appreciably during this period, 
and the amplification ratio of the cathode follower is very nearly unity; so that the 
potential of G^ follows quite closely the change in plate voltage during stage I, and 
the grid current to stops immediately at the start of stage I. 

During stage II the potential of Gj rises slowly toward B+ potential, and 
would do so with a time constant of RgC if the tube were not present. However, the 
voltage amplification from Gi to the plate causes the rise of Gj potential to be very 
much slower than it otherwise would be. The rise of Gj potential results in a fall 
of plate potential which is transferred by means of capacitor C back to the grid, thus 
reducing the net amount of its potential rise. The discharging of C through Rg takes 
place with an effective time-constant of approximately RgC where >4 (of the order 
of 1000) is the amplification factor of the phantastron tube. During stage II the 
plate potential drops quite linearly until at a certain point there is a rather sudden 
loss of amplification in the tube. Although the loss of amplification occurs sudden- 
ly enough to produce accurate timing of the cycle of events, the end of the cycle 
(which takes place in stages III, IV and V) contains no abrupt voltage changes com- 
parable to those in a multivibrator circuit. 

After the loss of amplification, the potential of G^ rises toward potential 
with unhindered, simple time-constant, RgC. The cathode potential follows the 
rise of grid potential with the screen grid taking most of the additional space cur- 
rent. The drop in screen grid voltage does not divert current to the plate circuit 
because the increasing negative bias of G 3 offsets this effect. Finally (at the end of 
stage m) the increased bias on G 3 starts to shut off the plate current quickly and 
causes the plate potential to rise. 

During stage IV the plate potential rises and the grid and cathode voltages 
also rise rapidly due to the regenerative effect. The screen- grid potential drops 
sharply since it takes the additional space current, and finally drops so low that the 
screen current no longer increases. At some point during stage IV the rise in 
cathode potential triggers the pulse amplifier of Figure 22-26. 

Stage V is the recovery stage in which the plate potential rises with time 
constant RlC stray until it is caught by the left hand diode at the control voltage 
level. The circuit is then ready to be triggered again. 

Delayed Sweep for Beacon Observation 

In order to avoid the use of an excessively long sweep when viewing responses 
from distant beacons, it is necessary to delay the start of the PPI sweep until the 
beacon signal has nearly arrived. With a shorter sweep length, it is possible to 
obtain greater precision of setting the adjustable slant- range marker to the beacon 
response signal. During beacon operation it is usually most convenient to use the 
20 - mile sweep, the start of which is delayed from the triggering of the transmitter 
by a time interval corresponding to any desired integral number (0 to 23) of ten- 
mile distances. The necessary wide-range time- delay in 10- mile steps cannot be 
determined to the desired accuracy by means of a phantastron circuit alone. A 


Airborne Radar 


22.21 


phantastron and gate generator are used 
to produce a voltage gate 8 to 1 2 miles in 
width and centered as nearly as possible 
at a time corresponding to the desired 
range-delay of n x 10 miles after the trig- 
gering of the "main bang" by one of the 
10- mile pips from the crystal oscillator. 
In normal beacon- operation the trans- 
mitter is triggered by every 32nd 10-mile 
pip. The delayed gate from the phantas- 
tron and the output of the 10- mile pip gen- 
erator are applied to the grid and cathode 
respectively of an amplifier tube biased 
well beyond cut-off (See Figure 22-29). 
Neither the positive gate on the grid nor 
the negative pulse on the cathode is alone 
sufficient to cause conduction of plate cur- 
rent. However, a 10- mile pip occur ing at 
any instant while the 8- mile gate is on the 
grid will cause a pulse of plate current 
through the tube. The negative pulse at 
the plate of the gated amplifier serves to trigger the "single-shot" blocking- oscil- 
lator, which in turn triggers off the range and altitude delay- circuits. 

The 8- mile gate is generated in a multivibrator circuit similar to that des- 
cribed previously for obtaining a 12 microsecond delay time. The circuit with wave- 
forms is shown in Figure 22-30 along with a triggering circuit controlled by the 
cathode- output voltage of the phantastron circuit. 

Combined Functioning of the System Components 

The combined functioning of the component parts of the system will now be 
discussed with the aid of block diagrams and timing diagrams. Figure 22-31 is a 
time diagram for the events in the range unit, transmitter, and indicators when the 
system is used for high-altitude search or bombing; and Figures 22-32 and 22-33 
are functional timing and block diagrams respectively of the range unit, also for 
the case when the system is used for high- altitude search or bombing. Referring to 
Figure 22- 33 the 10:1 and 7;1 frequency dividers following the one- mile pip genera- 
tor controlled by the crystal oscillator, 
allow every 70th one- mile pip to initiate 
a transmitter pulse. The repetition period 
is about 865 microseconds correspond- 
ing to a pulse- repetition-frequency of 
about 1155 cps. Since during high- alti- 
tude search or bombing, the PPI sweep 
is delayed by an amount corresponding to 
the altitude of the aircraft, the modulator 
is triggered by the cathode follower at the 
end of the chain of frequency dividers in- 
stead of by the delayed output of the timing 
oscillator . The instant at which the output 
of the cathode follower triggers the modu- 
lator is taken as zero reference time for 
Figures 22-31 and 22-32. In order to de- 
lay the PPI sweep by an amount corres- 
ponding to the altitude the adjustable- 
delay- phantastron shown in block dia- 



Fig. 22-30 Delayed trigger 
amplifier and 
selector-gate 
generator 



Fig. 22-29 Gated amplifier 
coincidence circuit 
and trigger blocking 
oscillator 


22.22 


Airborne Radar 



Fig. 22-31 Timing diagram for events in the range unit, transmitter, and 
indicators, when the system is used for high altitude search and bombinfr 


Airborne Radar 


22.23 


MiLCS 



Fig. 22-32 Timing diagram of range unit as used in high-altitude search and 

bombing 


22.24 


Airborne Radar 


ALTITUDE CHANNEL 



Fig. 22-33 Functional block diagram of the range unit during high- 

altitude search or bombing 

gram form in Figure 22-33 delays the triggering of the timing oscillator and 
sweep circuits by an amount which is 12 microseconds less than the time re- 
quired for an electromagnetic signal to travel from the aircraft to the ground 
and return. Although the "A" scope sweep starts immediately upon being triggered, 

20 MILE SWEEP IS SHOWN 
ALL TIMES IN MICROSECONDS 

TRANSMITTER PULSE 



Fig. 22-34 "Stop-watch” diagram of operating sequences in high-altitude search 

and bombing 


Airborne Radar 


22.25 


the PPI sweep does not get started until 12 microseconds after triggering as illus- 
trated in Figure 22-31. An altitude marker to be matched to the first ground return 
is generated 12 microseconds after the triggering of the sweep circuits and corres- 
ponds in time to the effective starting point of the PPI sweep (See Figures 22-31, 
22-32, and 22-33). A gate from the 12 microsecond delay circuit unblanks the PPI 
after the 12 microsecond delay. In Figure 22-31 both indicator and respective 
sweep signals are superimposed schematically on the vertical scale. The pip which 
marfe a portion of the precise slant- range marker-circle, at a given instant in each 
sweep, is generated in the range- pip blocking- oscillator, which is triggered by the 
adjustable 0.6 to 16 mile, range-delay phantastron. 

Other timing diagrams which may be helpful in visualizing the operating 
sequences used in high altitude search and bombing are shown in Figures 22-34 and 
22-35. 



Fig. 22-35 Block diagram of "stop-watch" correlation with the basic timing 
functions in high-altitude search and bombing 


Figure 22-36 is a functional block diagram of the range unit during naviga- 
tion by beacon, and Figure 22-37 is the corresponding time diagram. Figures 
22-38 and 22-39 are "stop-watch" and block diagrams respectively for the basic 
timing functions during navigation by beacon. The pulse repetition frequency is of 
necessity much lower than that used in high-altitude search and bombing because 
of the large time-delays required when observing beacons which may be as far 




22.26 


Airborne Radar 


BEACON delay channel 



Fig. 22-36 Functional block diagram of the range unit during navigation 

by beacon 


distant as 250 miles. 10:1 and 32:1 frequency dividers following the crystal oscil- 
lator allow every 320th one- mile pip to trigger the modulator. The PPI sweep 
is delayed a suitable number of ten- mile intervals by the trigger step- delay phan- 
tastron. The 10- mile pips (output of the 10:1 frequency divider) and the selector- 
gate output of the trigger-delay phantastron are mixed in the coincidence circuit in 
order to obtain crystal accuracy in the beacon delay time. The output of the coin- 
cidence circuit initiates the operation of both altitude and range channels as shown 
in the block diagrams. The calibrated range phantastron in the range channel 
allows the pip for the adjustable range marker circle to be delayed by an amount 
suitable for matching to the beacon signal. When the range marker circle is proper- 
ly matched to the beacon response signal, the range to the beacon is the sum of the 
ranges corresponding to the delays introduced by the beacon step- delay channel and 
the range channel. During navigation by beacon, the setting of the altitude phantas- 
tron is of no consequence since the matching of the range marker circle to the 
beacon echo does not depend upon the time at which the sweep starts. The operation 
of the altitude channel is the same as previously described except that during bea- 
con navigation, no use is made of the altitude mark appearing on the "A" scope. 


Bibliography 


Identification 

Classification 

Title 

Issued by 

135-c 

Confidential 

Handbook of instructions for 
radio set AN/APS-15 (H2X) 

MIT Rad. Lab. 

AN-05-15-16 

Restricted 

Gyro-Stabilized Flux- Gate 
Compass System 

^see below 


^ Joint authority of Commanding General Army Air Forces, the Chief of the 
Bureau of Aeronautics and the Air Council of the United Kingdom. 


Airborne Radar 


22.27 


320 


80.86 KC 

<1 NAUTICAL MILE) 
PIPS FROM CRYSTAL 
OSCILLATOR AND PIP 
GENERATOR 


8.088 KC 
(10 NAUTICAL MILES) 
PIPS FROM IO:i DIVIDER 


ENLARGED 
FIRST 80 MILES 



BEACON PHANTASTRON 
DELAY GATE 


8 MILE 

SELECTOR GATE 


DEUYED TRIGGER 


RANGE PHANTASTRON 
DELAY GATE 
iO.6 TO 16 MILESU- 


RANGE PIP 
TO PPI 


ALTITUDE PHANTASTRON 
DELAY GATE 


( 10,000- 36,000 FtT 


INDICATOR TRIGGER 


12 MICROSECOND DELAY 


PPI SWEEP 


BEACON 0 


ELAY 


MILES 


RANGE-MARK, DELAY- 


MATC 


5 MILES 


HED TO TAf^GET AT 75 MILES 


PREVIOUSLY SET AT PLANE*S ALTITUDE - 1 2 p SEC. 


(HERE 


3 NAUTICAL 

37- 12 = 25 


MILES— ISklSEC.) 

^sEc. r 


1 


PPI SWEEP STARTS 


Jl_ 




Fig. 22-37 Timing diagram of range unit as used in navigation by beacon 


22.28 


Airborne Radar 


ALL TIMES IN MICROSECONDS 



Fig. 22-38 "Stop-watch" diagram of operating sequences during navigation by 

beacon 



Fig. 22-39 Block diagram of "stop-watch" correlation with the basic timing 
functions during navigation by beacon. 


Airborne Radar 


22.29 


The H3X attachment for airborne radar 

H3Xis an H-type system similar to Micro-H. It is an X-band radar attach- 
ment that provides automatic range-tracking of two beacons giving out continuous 
range information. The computing circuits permit the flying of either a hyperbolic 
course or a cat- mouse course. H3X has been developed but since it is more compli- 
cated than Micro- Hand gives accuracy not significantly greater than that of Micro-H, 
it is no longer being worked on. 

Bibliography 

Identification Classification Title Issued by 

71-7/19/44 Confidential H3X (Centimeter-H); A MIT Rad. Lab. 

Beacon Blind Bombing Attach- 
ment for Airborne X-band 
Radar 


The NOSMO Attachment for AN/APS-15 

TheAN/APA-46(NOSMO) equipment is an attachment for AN/APS-15 or-15A 
airborne radar sets although it can also be used with AN/APQ-13 if a special con- 
trol-box is provided. By means of this attachment it is possible to use the radar 
information of target range and bearing to synchronize the Norden bombsight in 
range. This permits bombing with the Norden sight either in case the radar target 
echo should break up before reaching the bombing circle, or possibly to take advan- 
tage of any last minute visibility in which there would be insufficient time to syn- 
chronize the bombsight by normal optical methods. As a navigational aid, the NOSMO 
equipment provides for pulse-Doppler drift determination, which allows a quick deter- 
mination of drift angle and ground track without the use of a reference point and 
without turning the aircraft. Only the navigational function will be described here. 

In general, whenever an observer and a source of wave motion are either 
approaching or going away from one another, the number of waves received per sec- 
ond by the observer (i.e. the frequency of the received waves) is increased or de- 
creased respectively. The change in frequency is proportional to the relative velo- 
city between source and observer. 

Inthecaseof a moving aircraft transmitting and receiving radar signals, the 
change infrequency is multiplied by a factor of two because the waves make a round 
trip. The frequency received by an aircraft from radar echo-points lying anywhere 
along a line passing through a point directly beneath the aircraft and making an angle 

2Vg cos 0 

0 with the aircraft' s ground track is given by ft + where ft is the trans- 

mitted frequency, X is the wavelength corresponding to the transmitted frequency, 
andVgis the groundspeed of the aircraft along its ground track. The radar beam is 
of finite width (about 3^ for H2X sets) and each transmitted pulse has a finite dura- 
tion. Due to the very slow variation of the cos 0 function about 0 = 0, radar echos 
from all points lying within the 30 beam near 0 = 0 have approximately the same fre- 
quency; and hence the output of the non-linear frequency converter contains frequen- 
cies which produce a very low beat or rate of flutter of the video signal. On the 
other hand when the radar beam is at an angle with respect to the ground track, the 
variation in cos 0 within the 3^ width of the radar beam is considerably greater than 
when 0 = 0 so that the composite radar signal returned to the aircraft will contain 
components having a greater spread of frequencies. The eye can detect rapid flue- 


22.30 


Airborne Radar 


tuations in intensity at frequencies below about 20 cps, above which frequency the 
image appears blurred. The flutter of the video signals due to the Doppler effect is 
visible within a sector of about seven degrees on the PPI screen. Since a yellow 
filter makes it difficult to observe rapid variations in signal strength, a sector or 
wedge-shaped blue filter is used in order to eliminate the persistence of the tube 
in the region within which the Doppler measurements are being made. During the 
Doppler measurements the antenna spinner may be aimed or "search lighted" in any 
desired azimuth. If the radar beam is slowly and smoothly positioned in the vicinity 
of the ground track, it is possible to locate with an accuracy of + the position of 
minimumbeatfrequency or rate of flutter of the signal intensity. The effect is most 
clearly seen on homogeneous ground clutter and optimum sweep length is about 15 
miles. The appearance of the null varies somewhat with the terrain, and is not suf- 
ficiently clear cut for measurement over water. 

The position of the antenna spinner which gives the minimum amount of 
signal flutter indicates the direction of the ground track of the aircraft. Either a 
dark or light line may be caused to occur on the PPI screen at this ground track 
azimuth. The drift angle is then the angle between the ground track indicator and 
the lubber line which gives the heading of the aircraft, and this angle may be read 
directly upon a drift angle dial on a control box. 

The NOSMO attachment contains computing equipment necessary to provide 
for measurement of groundspeed by synchronous tracking of a clear-cut echo on the 
ground track of the aircraft. The AN/APA-46 (NOSMO) equipment, provides for a 
highly accurate determination of wind velocity by means of a double drift, double 
groundspeed, or drift and groundspeed diagram plotted on an army type E6B pocket 
computer. From about 30 seconds to a minute is required for this determination. 
The NOSMO equipment weighs about 50 lbs. installed. 

Bibliography 

Identification Classification Title Issued by 

M-227 Confidential Preliminary Handbook of Oper- MIT Rad. Lab. 

ating and Maintenance Instruc- 
tions for Model AN/APA-46 Air- 
craft Radar Equipment 

63-^17/45 Confidential Recommended Operational MIT Rad. Lab. 

Procedure for use of Nosmo 
Over a Complex Target 

Airborne Radar for Navigation 

An airborne radar set such as AN/APS-15 is suitable for navigation either 
by means of beacon reception or through recognition of familiar radar echos from 
the landscape below. Attachments such as NOSMO, Micro-H, and GPI increase the 
usefulness of an airborne radar as a general navigational aid; but undoubtedly lighter 
and more efficient airborne radar equipment could be obtained by an entirely new 
design incorporating all of the features of the above mentioned attachments. Naviga- 
tion by radar is simplest over terrain containing railroads, large steel structures, 
land- water boundaries, etc., which provide easily recognizable echo patterns. Navi- 
gation over very flat inland terrain from which few strong echos are obtained is in 
general difficult without the use of responder beacons or suitably positioned corner 
reflectors. The details of Map-PPI Superposition (Radar mapping) are discussed in 
Section 26. The accuracy, range, usefulness for special services, etc., of airborne 
radar is compared with that of other types of navigational aids in Section 31. 


Search Radar as a Navigational Aid 


23.01 


High- definition ground (or ship-based) search radar may be used as a 
navigational aid since it will give the position of all craft within range. It is possible 
for any craft within range to get a fix by communication with the radar station. 
Identification of the particular craft requesting a fix is somewhat of a problem. 
There are several ways that this identification can be accomplished. One of the 
simplest methods and one that requires no additional equipment is for the craft 
to execute a specified maneuver that the radar operator can identify. This is slow 
and inefficient and not workable when many craft are requesting fixes. In many 
applications this is not a serious problem since the craft is continuously tracked 
or the track of many craft may be plotted on a plotting table. If the craft carries 
a responder beacon the problem of identification is somewhat simpler since some 
type of beacon coding may be used to identify the craft requesting identification. 
It appears that the most satisfactory coding is range coding. The beacon in the 
craft gives the further advantage of greatly increasing the range of the radar set. 
Two other systems that are covered elsewhere in this report are possible. One 
involves the relaying of the ground PPI presentation to the craft within range. Thus 
the navigator or pilot in each craft sees the same PPI presentation that the ground 
radar operator sees. He can identify his own response on the oscilloscope by any 
of the methods mentioned above. This system is described further in section 25 
on the Federal Traffic control system. The second method involves the 
transmission of the ground PPI presentation to the craft by television. In this 
system it is also possible to transmit a map of the region and much additional 
information to the craft. This system is further described in section 27 on the 
RCA Televised Radar System. 

It is the purpose of this section to describe briefly two of the typical radars 
that are suitable for navigational purposes. The most successful radars to date 
for the purpose are the MEW (AN/CPS-1) and SCR 584. Of these two the MEW has 
the greatest range and versatility. The MEW was originally designed as a micro- 
wave early-warning radar. Its great range and high definition have ideally fitted it 
for navigation and fighter-direction purposes. 

MEW (AN/CPS-1) 


Type of system 

Combination range and azimuth system. 

Useful range 

Maximum range (theoretical 266 miles). 

Single large aircraft at 20,000 to 30,000 feet - 175 miles. 

Single small aircraft at 10,000 feet - 100 miles. 

Smallest aircraft can be seen up to radar horizon if equipped with beacon. 
Minimum range - 1/2 mile. 

Accuracy and precision 
Range + 1 mile 

Azimuth +1® 

Resolving power 
Range 1/2 mile 
Azimuth 1/2^. 


23.02 


MEW (AN/CPS-1) 


Presentation - visual 

PPI scope, B scope, off-center PPI. 

Operating skills required 

Trained operators to interpret scope presentation. Time required to get 
fix - Position available continuously on long persistence scopes but is only correct 
when beam swings by aircraft. Rotational speeds of beam of 1, 2, and 4 revolutions 
per minute are available. 

Equipment required 

Weight - about 66 tons (crated for shipment). 

Complexity - This is one of the most complex radars made, both as to the 
complexity of individual circuits and the number of circuits used. 

Service and maintenance requirements - highly trained personnel. 

Radio- frequency spectrum allotments required 

Frequency - 2700 to 2900 mcps. 

Wavelength - 10.3 to 11.2 cm. 

Bandwidth - 3 to 4 mcps. 

Present status 

Operational. 

The MEWis a long-range microwave search-radar system. Its high angular 
definition is obtained by the use of a very large antenna system. The antenna 
system consists of two cylindrical parabolic dishes placed back to back. These 
are fed by arrays of 106 dipoles fed by a wave guide. One of these dishes gives 
a low- angle beam for detecting low-flying aircraft or aircraft at long ranges. The 
other dish gives a high- angle cosecant-squared beam for detection of near, high- 
flying aircraft. The half- power beam-width of both beams in the horizontal plane 
is 1.5^. The low-beam dish is 25 feet long and 8 feet high and the high-beam re- 
flector is 25 feet long and 5 feet high. Experimental dishes 50 feet long have been 
built. Each dish has two transmitters and receivers. Each transmitting magnetron 
is on a slightly different frequency. The two transmitters in use are driven in 
parallel by a single modulator. 

The indicating equipment normally furnished consists of five 12-inch PPI 
scopes and five 7-inch B scopes. One 5-inch expanded A range scope mounted on a 
dolly is provided. This can also be used for general servicing. 

The PPI scopes have 60; 80; and 100-mile sweeps and have a variable delay 
adjustable from 0 to 200 miles. 10-mile range circles with every fifth one being 
intensified and 10^ azimuth markers with every third one wider are provided. A 
recent modification is the provision for off- centering the presentation. It can be 
off-centered by as much as 2 radii. 

The B scopes can cover an azimuth sector of 40® to 100® and have 10® 
azimuth markers. The sweeps cover 30, 80, and 100 miles with a delay from 0 
to 200 miles. 10-mile range markers are provided. 

The A scope has sweeps of 5, 50 and 200 miles. Any of these scopes can 
present the signal from either the high-beam antenna or the low-beam antenna. 


MEW (AN/CPS-1) 


23.03 


Ground clutter (the reflections from fixed reflecting objects on the ground) 
is always a problem in any microwave radar using a low angle beam. A method of 
decreasing this ground clutter has been devised. MTI (moving target indication) 
lessens ground clutter by suppressing the responses from fixed targets. Targets 
having a radial component of velocity will produce a doppler -effect change of 
frequency in the reflected signal. In order to make practical use of this effect 
(which is quite small), a beat method is used. The radar receiver must employ a 
very stable local oscillator. A beat oscillator working at intermediate frequency 
is used. This is rephased by every transmitted pulse. This is necessary since 
there is no consistent phase relation between successive transmitted pulses. With 
these modifications the response from stationary targets will be constant and those 
from targets with a radial component of velocity will flutter. That is, the pulses 
will vary from positive to negative at the beat frequency rate. In order to make 
effective use of this moving target flutter some sort of storage device is necessary. 
A liquid delay line has proven very effective for this purpose. Electronic storage 
tubes may also be used to store the responses from a transmitted pulse. The de- 
lay of the line is made equal to the pulse repetition period. The delayed signal 
from this line is mixed in opposite phase with the output from the receiver. The 
response from a fixed target will therefore be cancelled out since it is of constant 
amplitude. The response from a moving target will vary from pulse to pulse and 
will therefore not be completely cancelled out. An MTI modification kit for the 
MEW is being developed. 

Two new methods have been developed which deal with the method of 
presentation. Photographic Projection PPI (called P^I) consists of photographing 
the PPI scope, developing the film and then projecting an enlarged image on a 
screen from the film. This process has been developed to the point where the film 
can be exposed for one revolution of the PPI and then processed and be ready for 
projection in 10 seconds. If the antenna is making one revolution per minute 
an exposure can be made for each rotation. The film provides a permanent minute- 
to-minute record. The film may be projected in reverse on an 8-foot translucent 
screen. It can therefore be viewed from the side opposite from the projector. Plot- 
ting may be done directly on this screen. 

In many applications it is desirable to have a simple map or check points 
superimposed on the PPI presentation. This has been done by marking directly on 
the face of the scope with a china pencil. This method has the disadvantage that 
the scale or sector cannot be shifted without voiding the marking. An electronic 
method of superimposing a map and reference marks on the PPI presentation has 
been developed. The map to be superimposed is scanned radially by a beam of 
light in synchronism with the radar pulse rate and antenna rotation. The reflected 
light is focused on a photocell and the signal from this cell is amplified. This 
signal is mixed with the video signal from the radar receiver. After the initial 
registration has been made, change of sweep range, centering, or sector presentation 
will not affect the map superposition since it will move with the scope presentation. 
In effect this is really a television technique. 

SCR 584 


Type of system 

Combined range and azimuth. 


23.04 


SCR 584 


Useful range 

Maximum search range - 40 miles. 

Maximum tracking range - 1 8 miles . 

Minimum range .28 mile. 

Accuracy and precision 

Automatic tracking accuracy 
Range +15 yards (.0085 mile) 

Azimuth + .034 
Elevation + .034° 

Search accuracy 

Range + 1500 feet. 

Azimuth + 2^. 

Presentation 

Search - Visual PPI 

Tracking - Azimuth and elevation dials. 

Aided manual range tracking on J- scope (circular sweep), dial indication. 

Operating skills required 
Trained operators. 

Time required to get fix: time to read PPI when searching. Time to read 
dials when tracking. 

Equipment required 

Weight - 10 tons (installed in trailer) 

Equipment is quite complex. 

Service and maintenance requirements - highly-trained personnel required. 

Radio-frequency spectrum allotments required 

Frequency - 2700 to 2900 mcps 

Wavelength - 10.3 to 11.2 cm. 

Bandwidth - 3 to 4 mcps 

Present status 

Operational. 

The SCR 584 is a microwave radar designed for anti-aircraft gun-laying. 
It automatically tracks an aircraft in elevation and azimuth and has aided manual 
tracking in range. It can also be used for general search. It is not very well 
suited for general search since its beam is very narrow in a vertical plane as 
well as in a horizontal plane. It scans a 20^-elevation sector by using a helical 
scan. In other words, the elevation angle of the beam increases as the antenna 
rotates through 6 revolutions. It takes about one and a half minutes to make one 
complete scan. If it is desired to automatically track a selected echo the automatic 
search is stopped and the antenna is set on the echo manually by watching the PPI 
scope. 


TheSCR 584 is not well suited to following the movements of many individual 
aircraft or groups of aircraft but it is well suited to tracking one aircraft or group 
of aircraft. 


SCR 584 


23.05 


The indicating equipment consists of one 7-inch PPI with sweeps of 35,000 
yards (20 miles) and 70,000 yards (40 miles). Range -marker circles spaced 

10.000 yards (5.7 miles) apart are provided. An azimuth scale is provided around 
the edge of the PPI tube face. Range is measured accurately by the use of two 3- 
inch J scopes. One of these is the coarse range scope. One turn around the circle 
on it equals 32,000 yards (18.2 miles). One turn around the fine range scope equals 

2.000 yards (1.14 miles). The azimuth and elevation angle of the beam is indicated 
on two respective dials. 

An MTI modification kit is being developed for the SCR 584. 


23.06 


Search Radar as a Navigational Aid 


Identification 

Classification 

Title 

Issued by 

TMll-1544 

Confidential 

Radio Set AN/CPS- 1 

Service Manuah Theory, 
Trouble-Shooting, and 

Repair 

War Dept. 

TMll-1524 

Confidential 

Radio Set SCR- 584 

Service Manual: Theory, 
Trouble Shooting, and 

Repair 

War Dept. 

Radar" No. 10 
30 June 1945 

Secret 

Off-center PPI pp.20 
and 21 

Kits for the MEW 
pp. 55-57 

OSRD under 
direction of the 
Air Communi- 
cation Officer 


AN/APN 34 (Short-Range Approach) 


24.01 


Type of system 

Combines range, azimuth and DF (homing) on aircraft. 

Useful range 

100 miles (line of sight). 

Accuracy and precision 

Not known. 

Presentation 

3 meters indicating range (distance), homing and track. 

Operating skill required 

(a) At ground or fixed installation: can operate unattended. 

(b) In the navigated craft: very little skill required. 

(c) Time to obtain readings: instantaneous 

Equipment required 

(a) At ground beacon: Fairly complex transponder beacon and two -lobe 
antenna system. Highly trained personnel to service beacon. 

(b) In the navigated craft: Fairly complex interrogator- responser; highly 
trained personnel to service equipment. 

Radio-frequency spectrum allotment required 

Frequencies around 220 mcps. About 4 or 5 mcps bandwidth required for 
interrogator and beacon. 

Present status 

Experimental. 

Description of system 

This equipment supplies three types of information. It measures the range 
(distance) from the aircraft to the beacon, it determines if the aircraft heading 
points to the beacon; and it determines if the aircraft is on a given track. 

The equipment on the craft is essentially an interrogator- responser using 
a two-lobe antenna system and lobe switching. The equipment on the ground is a 
responder beacon feeding a two-lobe antenna system. The beacon responses are 
switched from one lobe to the other at a switching frequency of 10 cps. The pulse 
length is varied for the two lobes, long pulses being sent out from the right lobe 
and short pulses from the left lobe. In the aircraft equipment the responses from 
the right and left lobes may be separated by two pulse-len^h discriminators. These 
pulses may be integrated and applied to a left- right meter to indicate track. An 
automatic range-tracking follow-up system will stay locked on to the returning res- 
ponses and provide range information. A manual range-search must be used ini- 
tially to lock the follow-up on the desired beacon response. The lobe switching at 
the aircraft is done at a much higher rate than at the ground beacon. The output 
of the receiver is switched in synchronism with the lobe-switching and applied to 
a differential meter which indicates homing angle. 

Figure 24-01 illustrates the system. Aircraft 1 is headed toward the bea- 
con and therefore the amplitudes from its two lobes are equal. It is not on the 
track however and therefore the long pulses have a greater amplitude than the short 
pulses. Aircraft 2 is on track so the long and short pulses have equal amplitude. 
The aircraft is not headed toward the beacon though, and therefore the amplitude of 
the response from the right lobe is greater than the amplitude of the response from 
the left lobe. 


24.02 


AN/APN 34 (Short-Range Approach) 



Fig. 24-01 


Federal Airport Traffic- Control System 


25.01 


Type of system 

Combined range and azimuth (Radar). 

Useful range and coverage area 

Line of sight. 

Accuracy 

Not known. 

Type of presentation 

PPI presentation on cathode-ray tube centered about ground installation. 
Self-identification of craft provided; PPI presentation on ground. 

Operating skill required 

At ground station: Highly trained operators to maintain search radar plus 
additional complex equipment. On craft: Operational training in interpretation 
of PPI presentation. Little technical skill required. Time to obtain a fix: Instan- 
taneous. 

Equipment required 

At ground station: Microwave search radar, UHF pulse transmitter and 
receiver, and fairly complex control circuits. In craft: Microwave receiver, UHF 
receiver, UHF pulse transmitter, cube-law sweep-curving circuits and cathode-ray 
tube circuits. 

Radio-frequency spectrum allotments required 

Frequency: Microwave channel - 3,000 to 10,000 mcps. 

UHF channel - 500 to 1,000 mcps. 

Wavelength: Microwave channel - 3 cm. to 10 cm. 

UHF channel - 30 cm. to 60 cm. 

Bandwidth: Microwave channel - 6 to 8 mcps. 

UHF channel - 4 to 6 mcps. 

Present status 

Proposed. 

Description of system 

This system makes use of two different types of radar systems, simultan- 
eously. Since these systems could work separately it is simpler and more convenient 
to describe them's eparately. The two systems used are the three-path radar, here- 
after referred to as 3PR and the rotating lighthouse system, hereafter referred to 
as RLS. 

In the 3PR system a powerful microwave search-radar transmitter and 
antenna are used. All planes are assumed to be equipped with responder beacons 
consisting of a microwave receiver and an UHF pulse-transmitter. As the micro- 
wave search-radar beam sweeps past a given plane its beacon responds to each 
microwave radar pulse by transmitting an UHF pulse. At the ground station this 
UHF pulse is received by the UHF receiver. The output of this receiver triggers 
off the microwave pulse-transmitter which radiates these echo responses omni- 
directionally. Transmitted simultaneously with each directional radar pulse is 
an omnidirectional UHF synchronizing pulse. Figure 25-01 represents the ground 
station and two aircraft. At the moment represented the search-radar antenna 
is directed toward aircraft 2. Let us consider the signals that are received at 
aircraft 1. At the ground station the microwave radar pulse and the omnidirection- 


25.02 


Federal Airport Traffic^Control System 



Ground 

Station 


Fig. 25-01 Principle of three-path Radar . 


al UHF synchronizing pulse are simultaileously transmitted. The omnidirectional - 
UHF pulse travels the distance d^ and arrives at aircraft 1 at the time t^^. . The 
microwave radar {iulse travels the distance d 2 to aircraft 2.^ The beacon in 
aircraft 1 responds with an UHF response pulse. This UHF pulse travels the 
distance d 2 back to the ground station and arrives there at a time t 2 ; at the 
ground station the UHF response is retrans^litted as an omnidirectional jmicro- 
wave pulse. This experiences an additional rfelay and a|*rives at aircraft 1 at 
a time t^ + 2t2. Thus the difference in time of arrival at aircraft 1 of the UHF 
synchronizing pulse and the omnidirectional mici:owave pulse is (tj + 2t2) • 

2t2. This is the same delay that is observed, at the ground station between the 
directed microwave radar pulse and the UHF response pulse. If the cathode-ray 
sweep in aircraft 1 is started by the UHF synchronizing pulse then the omnidirec- 
tional microwave pulse will be displayed at the correct point to form^ a PPI presen- 
tation centered about the ground station. One other requirement must be met to 
produce a true PPI presentation on aircraft 1. The rotating deflection system of 
the cathode-ray tube must be synchronized with the rotating radar antenna at the 
ground station. This is accomplished by having the UHF synchronizing-pulse trans- 
mitter transmit a special pulse of different width as the radar antenna .i5 wings through 
north. 

Since this radar system receives beacon responses on a different frequency 
than its own pulses it will "see" only responder beacons. Natviral obstacles, run- 
way ends, etc. can be indicated by the use of ground beacons. All aircraft within 
range including aircraft 1 itself will be shown on aircraft I's PPI. Since there is 
no difference between the aircraft* s own response and that of other aircraft on the 
PPI,, self-identification is not provided. It would be possible to identify your own 
plane by interrupting your beacon* s response for a few seconds and noting which 
spot disappears from the PPI. 

\ 

In the RLS (Rotating Lighthouse System) a high-f)owered microwave pulse 
transmitter feeds a rotating directional antenna. This can be the same combination’ 
as used in the 3PR system. The function of this microwave system is to illuminate 
all objects within range of the ground station. 

In Figure 25-02 the ground station, two aircraft, and a natural obstacle are 
represented. The omnidirectional UHF synchronizing pulse and the microwave 


Federal Airport Traffic-Control System 


25.03 



Omnidirectional U.H.F. 
Synchronizing Pulse > 


Ground 

Station 


Fig. 25-02 Principle of Rotating Lighthouse System 


radar pulse are emitted simultaneously. The UHF synchronizing pulse travels the 
distance dj to aircraft 1 and upon arriving there starts the sweep. The micro- 
wave radar pulse is assumed to be directed out toward aircraft 2 and the natural 
obstacle at the instant represented. The microwave radar pulse travels the dis- 
tance d 2 to aircraft 2 and triggers off the responder beacon which radiates an 
omnidirectional UHF pulse. This pulse travels the distance d^ to aircraft 1. The 
microwave radar pulse also travels the distance d 3 to the natural obstacle and 
some of this energy is reflected to aircraft 1 along the path d 5 . Thus at aircraft 1 
beacon responses may be received on an UHF receiver and direct reflections may 
be received on a microwave receiver. This system is therefore equivalent to a 
radar system in which the receiving equipment is at a distance from the transmitter. 
If the rotating deflecting system of the cathode ray tube of aircraft 1 is kept syn- 
chronized with the direction in which the radar antenna is pointing then the angular 
indication on aircraft 1 ' s PPI will be correct if this PPI is centered about the 
ground station. The directional indication will be correct since only those objects 
in the path of the radar beam can produce responses. In order to give correct dis- 
tance indications a nonlinear sweep must be used on the cathode ray tube in aircraft 
1. The shape of this sweep is a function of the distance dj and the angle p. The 
angle p may be determined from the time when the microwave radar beam sweeps 
past the aircraft. The distance dj may be determined by the three path radar in- 
dication. For a small angle p as shown the sweep would move the cathode ray spot 
very rapidly at first and then slower. (See Figures 25-03 and 25-04). The fact 
that di and p must be known to give a correct RLS indication means that the self- 
position of the plane must be known. By combining this RLS with the 3PR system 
a valuable check is obtained since the 3PR system does give a presentation correct 
inazimuth and range. It is proposed to use alternate pulses of the microwave radar 
transmitter for the RLS and 3PR functions. The two presentations would be super- 
imposed on, the same cathode ray tube. Figure 25-05 gives the appearance of the 
RLS display. Self-position is indicated by the ellipse. The end of the ellipse at the 
center of the PPI indicates the ground station and the outer end indicates self- 
position. The ellipse is a blind area. No objects can be seen in it by the RLS 
function. This is not serious however since the 3PR system will give indications 
of planes in this area. 


25.04 


i 


Federal Airport Traffic-Control System 


o 

UJ 


4 



o 

ui 

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Fig. 25-03 Sweep waveforms for aircraft Fig. 25-04 Sweep waveforms for aircraft 

at 9 miles from lighthouse at 3 miles from lighthouse 


Federal Airport Traffic-Control System 


25.05 



Fig. 25-05 Rotating Lighthouse Presentation 


Identification Classification Title Issued by 

Proposal No. Confidential Three-path Rotating Lighthouse Federal Tel- 

287 System for Airport Control ephone and 

Radio Corpor- 
ation 








Map“PPI Superposition (Radar Mapping) 


26.01 


Part I General Information 

The signal pattern on a PPI is a map-like presentation of echo signals re- 
ceived from surrounding artificial and topographical features of many kinds. Even 
though the radar set be located at a considerable altitude above the earth' s surface, 
by proper design and adjustment of the PPI sweep circuits the echo signals may, 
with a high degree of accuracy, be made to appear at the correct ground range and 
azimuth from the radar set. With the possible exception of a small area at the cen- 
ter, the relative positions of all signals with respect to one another are shown on a 
PPI with sufficient accuracy to enable a chart of the same territory to be fitted by 
superposition to the PPI signals. A navigational fix is thus obtainable from the posi- 
tion of the center point of the PPI upon the superimposed chart. 


Direct superposition in the form of an overlay is undesirable because of par- 
allax (due to thickness of the glass in the tube face) and distortions produced by 
electrostatic charge on the operators hands. Successful superposition may be accom- 
plished by an optical method which produces a virtual image of the chart in the plane 
of the cathode- ray tube face. One of the simplest forms which the apparatus may 
take is shown in Figure 26-01. Such an arrangement is called a Virtual PPI Reflect- 
oscope, usually abbreviated as VPR. The VPR apparatus does not make use of any 



Fig. 26-01 Essential parts of the VPR 


lenses and it is enclosed in a frame which 
also acts as a light- hood. The map or 
chart to be used is placed upon the illum- 
inated chart table at P. Rays of light from 
the chart are reflected from the mirror 
at R, and at D from the unfilmed side of 
the glass plate G. The operator sees the 
chart as a virtual image in the plane of the 
cathode- ray tube face (distance PR + RD + 
DC is equal to the distance PE). The ob- 
server also sees the PPI tube face through 
the filmed glass without loss of clearness. 

The chart which is prepared with 
white lines on black is readily shifted 
about by hand, and the chart table is also 
movable in its plane by two screws at right 
angles to one another. 


In some types of shipborne radar equipment 6 and 12- mile sweeps are avail- 
able on the PPI tube and two corresponding sizes of charts must be provided for 
superposition. The charts to be used are prepared in advance of their operational use 
and may have characteristics and markings of special usefulness for matching pur- 
poses. 


35 mm. microfilm may b'e used instead of a paper chart. The NMP (Naviga- 
tional Microfilm Projector) apparatus is similar to that of the VPR just described 
except that the chart table is replaced by a diffusing screen which receives the image 
of the map by optical projection from below. 

Another type of projector called the Autofocus Microfilm Projector has an 
adjustable range of magnification. This arrangement enables a chart to be matched 
to the sweep length of a number of different radar sets. The essential parts of the 
Autofocus Microfilm Projector are shown in Figure 26-02. 

As manufactured by the Spencer Lens Co., the apparatus has a few additional 
feat\ires. For measurement of azimuth, the shadow of an adjustable protractor may. 


26.02 


Map-PPI Superposition (Radar Mapping) 



Fig. 26-02 Essential parts of the 
Autofocus microfilm projector 


if desired, be cast upon the projection 
screen with red light which does not ap- 
preciably dilute the normal white light il- 
lumination of the map. The relative posi- 
tion of the PPI screen and the superimpos- 
ed chart is adjustable in the north- south 
and east- west directions by means of two 
micrometer screws carrying reset counters 
which can be set to zero at any convenient 
reference point on the chart which may be 
desired as an origin of coordinates. The 
counters read the x and y coordinate dis- 
tances to the nearest hundred yards, with 
the possibility of estimation to the nearest 
50 yards. The information on the coimters 
is given by the radar operator to the navi- 
gator who works on standard size naviga- 
tional charts. 


Still another type of projection sys- 
tem somewhat similar to the VPR is under development in which the image of the 
PPI scope is projected onto the table top upon which the navigator works. 

In the various types of Map-PPI superposition equipment, azimuth 
stabilization is almost always used so that true north is as the top of the PPI screen. 
After the radar operator has matched the chart to the PPI pattern in a manner con- 
sistent with the topography of the area and straight line propagation of the radiation, 
a navigational fix may be obtained by any of several methods. With the Spencer 
Autofocus type of equipment, a navigational fix may be obtained with the shadow 
protractor alone by taking bearings on any two known points of the chart. Alter- 
natively a fix may be obtained by measurement of both bearing and range of a sin- 
gle known point on the chart with the aid of the protractor and range marks. A fix 
may also be obtained from the readings of the north- south and east- west counters. 
In the VPR system a fix may be obtained by reading off the position coordinates of 
the center of the PPI scope upon whatever grid of coordinate lines is reflected with 
the chart upon the screen of the cathode- ray tube. 

Television techniques are utilized in a new electronic method of Map-PPI 
superposition. The map and reference marks to be superimposed upon the radar 
signals are scanned radially by a beam of light which is synchronized with the 
radar pulse rate and antenna rotation. The reflected light is detected by a photo- 
cell and amplified. This signal is then mixed with the video signal from the radar 
receiver and the two sets of signals appear superimposed upon the PPI. After the 
initial registration has been made, change of sweep range, centering, or sector 
presentation will not affect the map superposition since it will move with the scope 
presentation. 

For presenting a scope picture to a large number of peop^, a Photographic 
Projection PPI (called P^I) has recently been developed, The P% method consists 
of photographing a PPI presentation, developing the film, and then projecting an 
enlarged image upon the back of a translucent screen. Plotting may be done direct- 
ly on the frontside of the translucent screen. This process has been develop- 
ed to the point where the film can be exposed for one revolution of the PPI, and 
then processed so as to be ready for projection in 10 seconds. The processed film 
provides a permanent record of the operations . 

A PPI pattern may not bear too great a resemblance to a map because of 


Map-PPI Superposition (Radar Mapping) 


26.03 


antenna beam width, shadowing, 1/ effects, etc. Greater definition may be obtained 
by using narrower beams and shorter pulses. The gain of the receiver may be var- 
ied throughout the sweep so that equal targets at different ranges produce approxi- 
mately equal signals. The use of improved sweeps (linear for surface craft and hy- 
perbolic for aircraft) and either rotating coil or electrostatic PPl's help to eliminate 
map distortions . Little can be done about shadowing effects except to become fami- 
liar with expected PPI patterns, through previous experience and study of stereo- 
pair photographs or model relief maps. A good indication of expected PPI patterns 
may be had through a preliminary study of a model relief map, which may be illumi- 
nated from different angles with special flash lights. 

A number of topographical features show up on radar PPI scopes. Some of 
the features which are most useful in radar mapping are shorelines, islands, hills, 
etc. Groups of tall buildings have corner- reflector characteristics and give good 
reflections from all directions. Large lakes, lagoons, valleys, and rivers give use- 
ful blanks. 

The advantages of Map-PPI Superposition are: 

(1) The amount of quantitative information which can be presented graphically upon 
a small area is very great. 

(2) Appraisal by eye is quickest means of comparison of two sets of data of this type. 

(3) Radar signals cannot in general be effectively camouflaged or damaged. The 
navigator is not dependent upon the proper functioning of any person or appara- 
tus not aboard his own craft. 

Several sources of data for radar mapping are available such as the 1:80,000 
UJS. Geodetic Survey navigation charts (local mercator projections) and U.S. Geo- 
logical topographic maps. The best source of data for radar maps is an up-to-date 
aerial survey with complete "stereo- pair" coverage. The stereo-pairs show height 
on an exaggerated scale. Standard methods of photogrammetry are employed to pre- 
pare a radar map showing the features in their correctly projected positions. For 
sweeps of 12 miles or less the distortion of the mercator charts is not appreciable, 
but whenever possible, conic projection should be used. All charts issued by the 
UJS. Hydrographic Office are also available on microfilm. 


Part II The NALOC (Navigational Aids to Landing Operations Committee) System 

of Map-PPI Superposition 


Type of System 

Combination of range and azimuth (PPI presentation). 

Useful Range 

Approximately 10 miles for accurate radar mapping. 

Accuracy and Precision 

The location of the craft is known to within about +100 yards. 

Presentation of Data 

Visual presentation on PPI. 

Operating Skill Required 

A radar operator trained in Map-PPI Superposition (Radar Mapping); 


26.04 


NALOC 


Time for Fix 

Several minutes for an independent fix starting from scratch, but as many 
as 3 fixes per minute are possible when navigating on a course. 

Equipment Required on LCC 

(1) ShipborneX-band or K-band radar set with gyro compass for stabiliza- 
tion of PPI, VPR or NMP mapping equipment. 

(2) QBG sound head (underwater sound directional receiver). 

(3) Marine Odograph with reversed plotting head (dead reckoning recorder). 

(4) Recording Fathometers - t 3 rpes NK-2 and NJ-8. 

(5) Three type TCS radios for communications. 

(6) Type ZB/RU short wave radio directional receiver. 

(7) Miscellaneous equipment such as magnetic compass, clocks, etc. 

RF Spectrum Allotments Require d 

K-band or X-band. Bandwidths about 1-2 mcps. 

Present Status 

Operational. 

The NALOC system of navigation has been designed primarily for naval 
landing operations in which a highly technical vessel manned by skilled navigating 
personnel leads a wave of troop carrying vessels to within a few hundred yards of 
a target beach. The problem is to direct the wave of landing craft so that a landing 
may be made within + 200 yards of a target point on the beach in zero- zero visibility 
and with a range accuracy corresponding to an error of + one minute in time of 
arrival. 

The landing control craft is equipped with a number of navigational aids so 
that several methods of navigation are available for use in any landing operation. 
One of the most commonly used systems makes use of optically superimposing a 
map upon a PPI presentation. The details of this method have b^n discussed ear- 
lier in this section. As the superposition technique is used in NALOC, a fix is 
determined from the position of the center point of the PPI presentation upon 
the superimposed map. If VPR (Virtual PPI Ref lectos cope) type of apparatus 
is used, two sizes of charts are provided to match the 6- mile and 12- mile 
sweeps of the PPI tube. The charts used are prepared in advance of the land- 
ing operation and usually contain only information which is useful for matching it to 
the PPI pattern. As previously mentioned, the NMP (Navigational Microfilm Pro- 
jector) type of equipment uses 35 mm. microfilm instead of charts. 

There are several types of coordinate systems which may be more conven- 
ient to use in landing operations than ordinary latitude and longitude. The terminal 
objective or target point on the beach is often a convenient origin of coordinates. 
Two typical coordinate systems are shown in Figures 26-03 and 26-04. The range 
in Figure 26-04 designated in minutes is of course the actual distance divided by the 
normal speed of the operation. The particular network of coordinate lines used in 
an operation is laid out both upon the chart or film used in the projection system 
and upon the larger charts used by the navigator. 

For security reasons it is sometimes necessary to observe complete radar 
silence during all or the greater part of a landing operation, so other means of navi- 
gation must also be provided. The LCC (Landing Craft, Control) is equipped with 
an underwater-sound directional receiver (QBG sound head) to provide for navigation 
by means of sonic buoys. For landing operations, sonic buoys are laid one to two 


4130 


NALOC 


26.05 




26.06 


NALOC 


miles apart in a straight line about 10 miles offshore. The buoys are laid by sub- 
marine one to three days early and are timed so as to transmit coded signals for an 
eight hour period centered on H-hour, and then scuttle themselves. They are anchor- 
ed at least 125 ft. below the surface of the water and are designed to keep within a 
radiusof± 200 yards of the anchor. The sound gear along with a recording fathometer 
and odograph (dead- reckoning tracer) may be used for the entire landing operation, 
but they are more commonly used while navigating seaward of the buoy line, and the 
radar mapping method is then used for the final 10- mile run to the beach. 

Although the sonic buoys may usually be heard up to 10 miles or more, under 
very adverse conditions it may be impossible to hear the buoys at distances greater 
than 1500 yards, or roughly one mile. It is also difficult under some conditions to 
lay a line of buoys by submarine with very great accuracy. In such cases it may 
therefore be desirable to navigate entirely by radar. 

Since the range of some Map-PPI Superposition equipment is limited to about 
12 miles, navigation by suitably positioned corner reflectors may be used during 
the early part of a landing operation. Boats with coded corner reflectors may be 
anchored at suitable positions off the coast line. With the aid of such reflectors it 
is possible to navigate by means of "backward ranging" radar and thus preserve 
radar silence toward the shore until the last 10 miles of the operation when the radar 
may be beamed shoreward for navigation by radar mapping. 


Bibliography 




Identification 

Classification 

Title 

Issued by 

503 

Secret 

Precise navigation by means 
of a radar map superposed on 
the PPI 

MIT Rad. Lab. 

658 

Confidential 

A microfilm chart projector 
for radar navigation 

MIT Rad. Lab. 

No. 10 

June 1945 

Secret 

"Radar" magazine, page 57 

OSRD under the 
direction of the 
Air Communica- 
tions Officer 

NALOC Pro- 
gress Reports 
No. 1 - No. 7 

Secret 

Progress Report, NALOC 
to Chairman NDRC 

MIT Rad. Lab. 


RCA 


27.01 


RCA TELEVISION-RADAR SYSTEM 


Type of system 

. Combination range and azimuth. 

Useful range and coverage area 

Approximately 200 miles, depending on height of craft (Line of sight). Day 
and night coverage identical. 

Accuracy 

The accuracy of position indication is that given by the type of ground search- 
radar used. 

Type of presentation 

Visual, (a) At ground station, several PPI indicators, each covering a pre- 
determined altitude range; also inserted information including maps of airways, 
airports, data regarding weather, etc. (b) At craft, a televised image is presented 
which reproduces the ground PPI picture corresponding to the altitude range desired, 
superimposed on the inserted information. 

Operating skill required 

This system is intended for traffic control, collision prevention, blind- landing 
approach and general navigation, (a) At the ground station, a permanent staff in- 
cluding traffic controllers, PPI and television operators, power plant attendants, etc., 
would be required. The skill required is determined by the function of each member 
of the personnel, (b) In the craft, the only operations required are the tuning of a 
television receiver to the desired channel, the normal adjustment of intensity required 
with any cathode-ray indicator, and ability to interpret the composite picture seen, 
(c) A fix is indicated continuously and automatically by the position of the craft as 
given in the picture with respect to fixed ground objects, airport runways, etc. 

Equipment required 

(a) At ground station: ground search-radar set (MEW or SCR 584 anchor 
GCA), a number of PPI indicators, optical projection systems, television cameras, 
televisiontransmitters, plotting facilities, telephone and radio communications gear, 
etc. (b) In craft: beacon transponder with adjustable code, barometric altimeter 
(standard equipment), television receiver. Normal VHF communication equipment 
is a useful adjunct. 

Frequency requirements 

Two S- or X-band radar channels are required. The band-width depends on 
the degree of oscillator stabilization realized in the craft beacon transmitter and in 
the ground search radar transmitter. Several television channels are also required. 

Present status 

RCA has made proposals which outline the scheme, but so far as we are 
aware complete equipment design has not been worked out. Inasmuch as the system 
uses components which have already been developed, the amount of new circuitry 
to be developed is not too large. 

Principle of operation 

The block diagram of Figure 27-01 is functional only, and does not indicate 
a specific arrangement of the equipment. The ground search radar transmitter 
presents a PPI picture which is televised and transmitted to the craft where it is 
received and displayed. A second television camera enables a chart to be super- 


27.02 


RCA 


imposed on the presentation, so that airways, information as to wind direction and 
velocity, traffic control and other special instructions, data as to obstacles, etc., 
may be temporarily or permanently superimposed on the pattern. It will be seen 
that this arrangement is extremely flexible and that a pointer might momentarily 
be used to designate pictorially areas or objects which are the subject of conversa- 
tions over the VHF communications equipment. It is understood that television 
from ground to plane is well established and that the weight of an airborne television 
receiver may be kept within a reasonable limit. It is proposed that a projection- 
type PPI display be used on the ground, and an ordinary CRT display in the craft. 



Television 

Antenna 


(a) Ground equipment 



(b) Craft equipment 


Fig. 27-01 RCA Televised radar system- -Block diagram 







RCA 


27.03 


Separation of different altitude levels 

The beacon receiver in the craft picks up the search pulses from the ground 
station, and the beacon transmitter radiates its response. This consists of two 
pulses. The first of these is assumed to be radiated with negligible delay. The 
second is coded by the plane' s barometric altimeter. That is, the time interval be- 
tween the first and second beacon pulses is controlled by a delay-line which is inter- 
locked with the altimeter, the length of the delay time being a function of the height 
of the craft. It is proposed that the delay should be variable in steps, each step rep- 
resenting a certain range of altitude. The altitude ranges would overlap slightly. 

At the ground receiver, an identical delay-line is used, and signals with and 
without delay are mixed and clipped. This results in the automatic selection (at 
the ground station) of responses from all craft in the altitude range for which the 


Ground Search 
Pulse ^ 


Time (psec) 


0 i 20 

Sweep 
Starts' 


40 


60 


80 


100 


120 


A Beacon Response 


Craft A(AA=i5psec) 


Craft B (AB = 20psec) 

h - aa-H 

Bi, 


|B2 

1 



1 

■ Ab 



Craft c (Ac = 25jusec) 




-Ac — 

1 Cg 

_Ij 


Time (p sec) I40 

160 

180 

200 220 

240 

260 

Received signals 


'''1 1 

A dH 

Ao B 1 * 1 

1 ^ '! 


jCg 

I 

Received and (/^s 
Delayed signals 

I5psec) 

h— aa— H 

A 

h — A a— >1 

[* A A — {* 

,c\ 

— -{Bz 

i 


clipping Level 

A 

AB, BC, 

1 i 

BC, 

i 


Added and Clipped 


1 

1 1 1 i i 

Time(psec) 280 

300 

320 

k 340 1 360 1 

o 

-00 

to 

400 


selected 

Signal 


Spurious Returns 


Indicated 
Ronges (psec) 


160 167^ 177^ 185 


Actual 

Ranges (p sec) 


ABC 
160 175 185 


Fig. 27-02 Time relationships 


27.04 


RCA 


ground delay-line is set. This principle is illustrated in Figure 27-02, in which a 
time scale starting from the emission of a ground pulse is shown. It is assumed for 
purposes of illustration that the coding time (A) for the particular altitude of craft 
A is 15 microseconds and that the distance of the craft from the ground station cor- 
responds to a one-way transit time of 160 microseconds. At 320 microseconds and 
335 microseconds the return pulses are received. The delayed received pulses occur 
at 335 and 350 fj sec. The second direct pulse and the first delayed pulse coincide in 
time and add up, so that after clipping, the 320 and 350 microsecond pulses will not 
appear, the only signal transmitted to the PPI being at 335 microseconds. It will 
be seen that there can be no coincidence between the direct and delayed pulses from 
any one craft unless the delay time at the ground station is the same as that at the 
craft. Thus by selection of the appropriate delay line on the ground, responses from 
planes in a given altitude layer are selected and displayed on a particular PPI. 

In order for the correct range to be indicated, the sweep at the ground sta- 
tion indicator must also be delayed. This is easily accomplished by triggering the 
sweep with that part of the ground search pulse whic^has also traversed the delay 
line. Thus the range indicated for craft A would be — - =160 microseconds. 

However, there is a possibility of spurious responses. Craft B (range 175 
microseconds, delay 20 microseconds) and craft C (range 185 microseconds, delay 
25 microseconds) produce overlapping returns as illustrated by the dotted lines. 
It is seen that there are three spurious returns from the two extra craft: two of 
these returns give incorrect ranges and all three give incorrect altitude layers. This 
condition will only arise if a number of craft happen to be within the same cone (as 
seen from the ground) corresponding to the dimensions of the search beam, at the 
same time; and unless these craft maintain the same spacing for a period of several 
search sweeps, the returns will be erratic, fading and changing position. This con- 
dition corresponds to dense traffic, would be relatively rare and could probably be 
recognized; nevertheless it appears to be a disadvantage and will no doubt be elimi- 
nated by careful design if this system is to be developed. 

Disposition of ground stations 

In regions where this type of control and navigation is to be utilized, ground 
stations would be established in suitable locations, perhaps 100 - 200 miles apart. 
The overlay or chart televised at one station would include printed instructions at 
appropriate points at its outer edge for retuning the craft receiver to the frequency 
used in the next area of control. The pilot of the craft sees his position in relation 
toother craft in the same altitude layer and to ground terrain. It would appear that 
heavy traffic under conditions of zero visibility might be handled by this method. 

Identification 

In order -that the craft pilot may identify his own craft on the display, it is 
proposed that the pulses from the craft transponder be made to brighten the craft 
television display for a time corresponding to one or two television frames. This 
will in effect brighten the televised image of the PPI sweep while it is pointing in the 
direction of the craft. Thus the pilot will see a bright line pointing at his position. 
If there is only one craft response along this line, he has identified himself. If there 
are several craft at the same level and azimuth, several responses will be visible 
along this line and in this case more distinctive identification such as momentary 
transponder coding by depressing a push-button switch, might have to be provided . 

Adaptation of the system to blind approach 

Where facilities must be provided for giving a pilot a precise indication of 
glide path, a modification is proposed. The craft equipment remains as before, the 


RCA 


27.05 


display being televised. The ground station consists of a GCA (ground- controlled 
approach) radar set, or simplified GCA omitting the "talk- down” feature, located 
just to one side of the far end of the runway. Azimuth and elevation sector-scan PPI 
indicators are necessary, as well as television cameras and transmitting facilities. 

The normal displays on the separate azimuth and elevation indicators fed 
from the GCA radar are shown in Figure 27-03. GO is the desired line of azimuth 


0 



0 



Fig. 27-04 Fig. 27-05 

Modified elevation scan Azimuth and modified elevation 

scans superimposed 


27.06 


RCA 


approach, QP is the projection of the desired glide path in a vertical plane. The 
craft A is correctly navigating along the glide path; at a later time it will have ar- 
rived at A' . The arcs xx and yy in Figure 27- 03 are drawn for illustration only and are 
not part of the display. Craft B is too far to the right and too high, although at the 
same range as A. 

In Figure 27-04 is shown a modified elevation display, wherein the amplitude 
of the sweep is modulated by the magnitude of the angle of elevation of the search 
beam at any instant in such a manner that the arcs xx and yy in Figure 27-03 now 
become the straight lines x^xj and y^y;^ in Figure 27-04. That is, the length of the 
sweep trace in Figure 27-04 is larger than in Figure 27-03 when the sweep is above 
QP, and smaller when it is below. It is presumed that suitable circuits can be de- 
signed to perform this modulation. Note that the distance between a response pip 
and the apex of the elevation display will now be equal to the corresponding distance 
in Figure 27-03 (a) or Figure 27-03 (b) only if the craft has the correct elevation. 

By suitable rotation of Figure 27-04 about its apex (in the plane of the paper), 
and by superimposing Figure 27-03 (a) with Figure 27-04 (rotated). Figure 27-05 
(which is the display as seen by the pilot in the craft) is obtained. The display of 
Figure 27-03 is scanned by one television camera and that of Figure 27-04 (rotated) 
by another. Between the display of Figure 27-04 and its camera there is placed a 
cylindrical lens, so oriented that points in Figure 27-04 appear as horizontal straight 
lines inFigure 27-05. Point A in Figure 27-04 gives rise to the line X 2 X 2 in Figure 
27-05, and point A* to the line y2y2- course” indication is therefore the 

coincidence of the '^elevation” line X 2 X 2 and the "azimuth pip" A with a point on the 
glide path GO. If the, pilot navigates in such a way that this coincidence is always 
maintained as A proceeds throu^ A' towards O, he will remain on the desired glide 
path. Plane B, which is too high, will give rise to the line zz and the response B in 
Figure 27-05. The^ilot, identifying himself with the response B, sees that he is 
too high (above the horizontal line zz) and too far to the right. As he approaches 
the correct glide path, B will move to the left towards OG and will continue to rise, 
but zz will rise faster, coincidence being obtained as for craft A. The pilot therefore has 
continuous indication of his position in space relative to the desired glide path and in 
relation to the airport, and may navigate accordingly. 

This system is not a full blind- landing system. If visual contact is obtainable 
at (say) 50 to lOO feet altitude, the blind approach procedure is followed until this 
"drop-out" level is reached. Otherwise some other schefhe adapted specifically for 
blind landing must take over after the approach has been completed. The indication 
is like that of a crossed-pointer meter except that the "crossed pointers" move up- 
ward on the "meter" as the airport (top of the "meter") is approached. Furthermore 
the positions of other aircraft making blind approaches are always shown. Also pic- 
torial transmission of airport runways, obstructions, etc., can easily be added to 
the picture as transmitted from the ground. A number of aircraft can be making 
approaches at one time and all can be correctly indicated. 

t 

The two modifications here described (control in the vicinity of large air- 
ports, blind approach) seem well adapted to handle large amounts of air traffic if 
they can be realized effectively. The considerable expenses involved should be 
justified by the volume of traffic that can be controlled under any conditions of visi- 
bility. 


RCA 


27.07 


Bibliography 




Identification 

Classification 

Title 

Issued by 

Eng. Memo 
PEM-16C 

Confidential 

Preliminary Analysis of 
Radar Navigation Aids 

RCA 

Eng. Memo 
PEM-17C 

Secret 

Memorandum on Radar- 
Television System of Air 
Navigation 

RCA 


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SPERRY 


28.01 


Sperry Omnidirectional Range and Distance Indicator 


Type of system 

Combined azimuth and range (radial lines of position combined with circular 
lines of position). 

Useful Range 

Azimuth: 100 miles at suitable height (line of sight) 

Range: 24 miles without ambiguity 

Accuracy and Precision 

(a) Azimuth measurement, about i 3° (estimated). 

(b) Ambiguity in range at over 24 miles. No ambiguity in azimuth. 

Presentation and use of data 

Altitude and range would be given by dial indication. Electrical data would 
also be available for automatic navigation on a preselected course. 

Operating Skill Required 

(a) At ground installations , where all equipment is automatic, only monitoring 
and occasional checking would be required, (b) In the craft, the only operations 
necessary are the tuning of azimuth and range receivers, (c) Since continuous in- 
dications of distance and azimuth are given, a fix is constantly available when the 
craft is within 24 miles of the ground station. Between 24 and 100 miles, azimuth 
only is available. 

Equipment required 

(a) At ground station: 50 - 100 watt C-band transmitter for rotating beacon 
transmission, antenna with dish reflector. Rotation or phase- shifting gear. Omni- 
directional transmitter and antenna for phase reference. Beacon transponder for 
distance indication, (b) At craft: C-band receiver and specialized timing circuits 
and indicator for azimuth. Transmitter and receiver with specialized phase-match- 
ing and indicating circuits for range. 

Radio-frequency Spectrum Allotments Required 

One C-band channel (5000 mcps) for azimuth measurement and two other 
channels (frequency undetermined) for range. Frequency stabilization of transmitters 
and receivers is proposed, so that the bandwidth required for the C-band channel 
would be of the order of 250 kcps ; for the range channels considerably less . 

Present Status of Development 

The Sperry Gyroscope Company has made proposals which are here outlined. 
Although various units in the system have been developed to the flight- testing stage, 
the system as a whole has not been integrated, nor have methods and equipment been 
frozen to a particular design. 

Principles of Operation 

1 . Azimuth Indication: The ground C-band transmitter is to be crystal controlled, fre- 
quency- multiplier klystrons being used in the final multiplier stages . A frequency 
stability of 1 part in 10” is thereby obtained. This means that many channels can 
be used within a small part of this band. Part of the energy from this transmit- 
ter, modulated at 3 or 4 kcps, is radiated from the omnidirectional antenna and 
is keyed at some definite repetition rate. The remainder of the energy at this fre- 
quency is transmitted as a fairly wide beam from the directional antenna. This is 


28.02 


SPERRY 


arranged to give a directional pattern consisting of two intersecting lobes, modu- 
lated at different audio frequencies. The directional beam is caused to rotate 
uniformly in azimuth at a speed of perhaps 2 revolutions per sec., by mechani- 
cally rotating the antenna and dish. Alternatively, some scheme of electrical 
rotation may be used (phase shifting), in which case the rotation could be some- 
what faster. The keying of the omnidirectional transmission is synchronized to 
the rotation of the directional pattern, so that pulses from the omnidirectional 
antenna constitute a definite azimuth reference. 

At the craft, the indicator essentially measures the time interval between 
the arrival of the omnidirectional azimuth reference pulse and that of the rota- 
ting directional beam, which is marked by the sharp change in modulation fre- 
quency as the intersection of the two differently- modulated lobes sweeps by the 
craft. This time interval characterizes a definite line of position in azimuth. 
The method by which the time intervals are measured and translated into a meter 
indication has not been definitely determined. Several mechanical and electronic 
timers are available. 

2. Distance Indication : The craft transmitter sends out a signal modulated at 3750 
cps. This is received in the receiver section of the ground repeater and retrans- 
mitted (on a different frequency) by the transmitter section, the modulation being 
preserved. At the craft, this ground repeater response is received and the phase 
of the modulation in the received signal compared with that of the modulation in 
the original transmitted signal. The difference in phase gives information as to 
the distance from craft to ground station. 

Since a change in phase of 360° at the modulation frequency of 3750 cps 
corresponds to a craft-ground station distance of 24.8 miles, there will be an 
ambiguity in the indicated range at distances larger than this. The method of 
phase comparison proposed is by synchro, with a suitable error- voltage and 
follow-up system. 

It is proposed that this modulation and phase comparison should be effected 
for short periods of 1/50 second, once every second. Thus by means of suitable 
switching and filtering circuits, the same channel could be used for communica- 
tions purposes. 

It is understood that the various units involved in these proposals have 
been developed and tested. Detailed circuits and methods for integrating the 
system are, however, not available. The basic idea in the thinking of the Sperry 
Company regarding this and other electronic communications and navigation sys- 
tems is that of narrow channels, with transmitters and receivers accurately sta- 
bilized infrequency to permit multi-channel operation and to improve signal- to 
noise ratios. The system described is intended to be of moderate range, filling 
the gap between long- range systems (such as Loran) on the one hand, and special- 
ized glide-path and blind- landing systems on the other. It is understood that the 
equipment used would be closely coordinated with other communications and 
navigational- aid equipment carried by aircraft at the present time, or. proposed 
for future use. 


The AN/APA-44 Ground Position Indicator (GPI) 


29.01 


Type of System 

The GPI is an automatic dead reckoning computer the operation of which is 
checked by the tracking of a reference radar echo appearing on a PPI scan. The 
auxiliary radar system is a combination of range and azimuth prototypes. 

Useful Range 

Up to + 1000 miles from a reference point. 

Accuracy and Precision 

The accuracy of the AN/APA-44 Gro\md Position Indicator attachment for 
X-band or K-band search radars is + 3% 

Presentation of Data 

North- south and east- west rectangular coordinate information of position 
relative to a fixed ground reference point is presented on dials provided that pre- 
vious adjustments have been made to cause an electronic crosshair to stay on a refer- 
ence ra^r echo appearing on a PPI. 

Operating Skill Required 

Unless a separate radar operator is provided, the navigator must be skilled 
in the interpretation of a radar PPI presentation. The GPI dial indications give an 
instantaneous navigational fix. 

Equipment Required 

The AN/APA-44 ground position indicator attachment for AN/APQ-7 or 
AN/APQ-34 search radars weighs about 175 lbs. 

Radio Frequency Spectrum Allotments 

The GPI attachment is designed for either X or K-band search radars. 

Present Status of Development 
In production. 

The AN/APA-44 ground position indicator is a radar navigaUonal aid and 
blind-bombing device with which one may (1) navigate with precision to a predeter- 
mined reference point or target; (2) approach a target from any direction taking 
evasive action if desired to within a few seconds of the time of bomb release; (3) 
bomb a target whether or not it can be seen on the PPI screen. It is essentially an 
automatic dead- reckoning device which provides for a step-by-step method of navi- 
gation. 


The equipment consists of a combination of computers, a pilot direction in- 
dicator (PDI), a "time- to- go” meter, and other supplementary apparatus designed 
for use with the search radars AN/aPQ- 7 and AN/APQ-34. This combination of 
units illustrated by the block diagram of Figure 29-01 provides an electronic index 
for the PPI (the intersection of a circular slant range marker and a radial azimuth 
marker) such that the index automatically follows a radar echo across the face of 
the PPI once the index is set on the echo by means of ”fix” knobs. Any easily 
disting^shable radar echo of known location may be used as a convenient reference 
point for ground position coordinates. The aircraft' s airspeed vector is resolved 
into north- south and east- west components to which are added the corresponding 
components of wind velocity. The resulting components of ground velocity are in- 
tegrated with respect to time and added to the initial settings of the GPI to give the 
N-S and E-W position coordinates of the reference point relative to the aircraft. 
The position coordinates are indicated on counter dials. These coordinates are 


29.02 


Ground Position Indicator (GPI) 


A N-S (FIX KNOB) 



Fig. 29-01 Functional block diagram of the GPI as used for navigation 


converted into polar form to give the ground range and true bearing of the reference 
point from the aircraft. The ground range is combined with altitifda^nformation to 
control a slant range ring on the PPI. The true bearing information controls an azi- 
muth mark on the PPI. If the correct wind data are set in, the electronic index will 
follow the reference radar echo on the PPI. If the marker drifts off the echo, the 
wind and position controls can be used to reset it at any time up to six minutes after 
the first fix. This automatically corrects the wind data, and the marker index should 
thereafter follow the radar echo unless there is a change in wind velocity. Even 
after the index and radar echo have moved off the face of the PPI (+ 20 mile limits), 
the dials continue to indicate the position of the reference point relative to the air- 
craft up to + 1000 mile limits. 

For step-by-step navigation, the electronic index on the PPI may be shifted 
to a new known reference point by proper resetting of the dials. The process can 
be repeated until the destination or target area is reached. 

For bombing, the index may be set either on the target or on a predetermined 
reference point for offset bombing. "Time- of- fall" and "trail" dials are adjusted 
for the proper altitude. Thepilot flies on two meters— the "pilot direction indicator" 
and the "time-to-go" meter. Evasive action may be taken until the "time-to-go" 
meter approaches zero. The bomb release may be automatic or not, as desired. 

The fundamental principles of operation of the Ground Position Indicator 
as used for navigation are discussed briefly. A block diagram of the GPI as used 
for navigation is shown in Figure 29-02, and one component circuit of the GPI is 


Ground Position Indicator (GPI) 


29.03 



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note: 


ELECTRICAL CONNECTIONS SOLID 
MECHANICAL CONNECTIONS DASHED 


Fig. 29-02 Block diagram of the GPI as used for navigation 


illustrated in Figure 29-03. Referring to the diagram of Figure 29-02, a DC voltage 
which is proportional to the craft' s true airspeed is impressed upon a sine potentio- 
meter. The position of the rotor shaft of the sine potentiometer is determined by 
the heading information from a gyrosyn compass system. The output of the sine 
potentiometer therefore consists of two DC voltages proportional respectively to the 
north- south and east- west components of airspeed. 

To each of these airspeed- component voltages is added a corresponding 
voltage proportional to the north- south (or east- west) component of the wind velocity. 
The two resulting voltages are thus proportional to the north- south and east- west 
components of ground velocity . These two groimd- speed component voltages are fed 
to rate- servos each of which yields a shaft speed proportional to the applied DC 
voltage. Ideally in a given time interval the amount of rotation of one of the rate- 
servo shafts is proportional to the net north- south ground travel of the aircraft, 
and the amount of rotation of the other rate- servo shaft is proportional to the total 
east- west ground travel during the same interval. In practice, minor corrections 
are introduced by the "displacement generator" described below. 

Figure 29-03 illustrates the addition of the east-west wind-speed to the 
east- west air-speed. This component circuit is a direct- current series loop, in- 
cluding a sine potentiometer, a wind- component potentiometer with its own floating 
voltage- source, a displacement generator with its shunting potentiometer, and a 




29.04 


Ground Position Indicator (GPI) 


FIX AND WIND 



rate-servo amplifier for the motor control circuits. The rate-servo motor drives 
the rate- servo generator at such a speed that its output voltage is just equal and 
opposite to the sum of the sine potentiometer, wind potentiometer, and displacement 
generator output voltages, to within a few millivolts. This small residual voltage 
appearingattheinputof the servo- amplifier serves to control the speed of the motor- 
generator combination. If at any time the voltage across the input terminals of the 
servo-amplifier exceeds a few millivolts, the motor rapidly accelerates or decelerates 
(depending on polarity) until the generator voltage is again almost equal to the sum 
of the other voltages in the loop. Thus the speed of the motor- generator combination 
is very nearly proportional to the sum of these voltages. Omitting, for the moment, 
consideration of any correction due to the "displacement generator", the motor- 
generator speed is therefore proportional to the east- west (or north- south) com- 
ponent of the aircraft’ s ground velocity, and mechanical coimters run by the motor- 
generator combination integrate the ground velocity to indicate the groimd mileage 
from the reference point. 

At the beginning of a flight the operator may know the approximate wind data. 
However, the components of wind velocity are seldom known with sufficient accuracy 
to enable the apparatus to correctly compute the aircraft's ground position to the 
desired degree of precision. Therefore, some means must be provided for check- 
ing and correcting if necessary the setting of the wind dials and simultaneously correct- 
ing the integrated reading of the ground- position indicator dials . This is accomplish- 
ed by tracking a reference radar echo with electronic slant- range and azimuth 
markers which are controlled by circuits in the ground position indicator. If the 
electronic index drifts off the reference echo due to incorrect wind data, the amount 
of drift in a given time interval is indicative of the error in wind velocity and may 




Ground Position Indicator (GPI) 


29.05 


be utilized to correct the error. 

"Memory-point tracking" is used to facilitate the correction of wind error 
so that the electronic index can be made to properly track a chosen reference radar 
echo. A manually- rotated DC generator (referred to as a "displacement generator" 
in Figures 29-02 and 29-03) is in each of the two component circuits. While being 
rotated, its output voltage adds algebraically to the ground- velocity component vol- 
tage, thus momentarily changing the rate-servo speed, resulting in a small correction 
of the electronic -index displacement and a corresponding change in the readings 
of the GPI dials. 

For convenience in the following discussion it is assumed that the setting of 
only one of the two component wind potentiometers is in error. When correcting for 
wind error, the amount of correction- shift introduced in a position component is 
proportional to the number of volt- seconds obtained from the displacement genera- 
tor during its rotation, and is therefore proportional to the amount of rotation. 

Across the terminals of the DC displacement generator is connected a linear 
potentiometer, the moving contact of which may be driven slowly away from its 
position of zero output by means of a constant speed motor. A given amount of 
generator rotation occuring at some time after the moving contact has left its zero 
position, produces an output in volt- seconds which increases linearly with the time 
elapsed since the potentiometer motion was initiated. Since the electronic index is 
drifting away from the reference echo at a rate proportional to the error of setting 
of the wind potentiometer, the total drift during a time interval is proportional to the 
interval. The amount of ground position shift (amount of rotation of the rate- servo) 
required for correction of the GPI mileage dials and the position of the electronic 
index, is proportional to the time elapsed since the moving contact left its point of 
zero output. This volt- second output, proportional to the elapsed time, is available 
at the output terminals of the attenuating potentiometer shown in Figure 29-03. A 
fixed amount of rotation of the displacement generator is required to produce the 
above mentioned output. Therefore, if the wind potentiometer is turned along with 
the displacement generator, at an appropriate ratio, the wind error may be elimin- 
ated by this rotation of the displacement generator. Therefore, further drift of the 
electronic index away from the reference radar echo will not occur unless there is 
a change in the wind velocity. Six minutes are required for the constant speed 
motor to drive the potentiometer contact over its entire range so that a wind cor- 
rection may be made at any time up to six minutes after making a setting upon a 
reference radar echo. 

When the "memory-point tracking" function is not being utilized, the circuit 
remains in its normal condition with the constant speed motor turned off and the po- 
tentiometers at their points of zero output. The track switches, however, are open 
(see Figure 29-03), so that the displacement generators offer maximum voltage, when 
required, for quickly setting the electronic index to a new radar reference- echo, a 
procedure which is carried out about every 15 to 20 miles during step-by-step navi- 
gation. 


Although the dials of the GPI read to+ 1000 miles from a chosen reference 
point, the electronic index on the PPI has a range limited to± 20 miles. 

The electrical connection from the GPI to the PPI index is controlled by 10- 
turn helipots (helical potentiometers) which are turned through reduction gears by 
the rate-servos. The rectangular- coordinate outputs of the GPI must be converted to 
polar form in order to control the range circle and azimuth marker comprising the 
electronic index which appears on the PPI. The rectangular to polar coordinate re- 


29.06 


Ground Position Indicator (GPI) 


solver circuit is illustrated in Figure 
29-04. It receives two AC voltages pro- 
portional to the rectangular coordinates, 
and delivers the polar coordinate informa- 
tion both in electrical and mechanical form, 
the radius or range information appearing 
as an AC voltage and the angle or azimuth 
data being given by the angular position of 
a rotor shaft. The GPI helipots are sup- 
plied with constant- magnitude 400 cps AC 
voltage of very low harmonic content. 
Transformers with grounded center taps 
are used so that the central or zero posi- 
tion of each helipot gives zero output vol- 
tage with respect to ground. North and 
south (or east and west) distances are dif- 
ferentiated from one another by signals of 
opposite phase. Voltages proportional to 
the north- south and east- west position 
coordinates are applied to the stator wind- 
ings of an Arma resolver which is a synchro 
having two windings in space quadrature on 
both rotor and stator. The stator voltages 
produce in this machine an alternating mag- 
netic field of strength proportional to the 
radial component and direction equivalent 
to the angle component of the desired polar coordinates. The voltage induced in one 
of the rotor windings is amplified and fed to a motor which turns the rotor until the 
coil is at right angles to the magnetic field andthe voltage picked up by it is zero. 
The other rotor coil is then aligned with the magnetic field and the magnitude of the 
voltage induced in it is indicative of the desired radius coordinate. The angle is 
accurate to within -0.6® and the radius or range voltage is accurate to within 1 40 yards 
in range measurement. 

The azimuth marker system illustrated schematically in Figure 29-05 com- 
bines information from the compass system, from the radar antenna spinner, and 
from the GPI resolver in such a way as to provide an azimuth mark with each ro- 
tation of the spinner. The following information is provided mechanically on shafts; 
(a) from the compass system: the true heading of the aircraft, (b) from the 
radar antenna spinner; the relative bearing of the spinner, (c) from the GPI re- 
solver: the true bearing of the electronic index. These three shafts are located in 
different parts of the aircraft, and electrical interconnection is accomplished by a 
differential- synchro system as illustrated in Figure 29-05 in which the dotted lines 
indicate mechanical couplings and the solid lines indicate electrical connections. 
Information from the compass and spinner is electrically combined to give the 
instantaneous true bearing of the antenna spinner. This information is compared 
with electrical information from a synchro on the GPI resolver giving the true bear- 
ing of the desired electronic index. When the above two directions coincide (i.e., 
whenever Or = 0c - Og), a radial azimuth marker is placed on the PPI screen. This 
occurs whenever the voltage from one of the terminals of the resolver synchro be- 
comes zero as at points A and C in Figure 29-05. This null is used in the azimuth 
marker circuit to produce an intensification of the oscilloscope beam. Another null 
in the voltage occurs at point B when the antenna spinner is pointing opposite to the 
desired direction. The different phase properties of the voltage output from another 
terminal of this autosyn are utilized in an appropriate circuit to suppress undesired 
marks corresponding to point B (see lower wave form of Figure 


MOV. (ilO%) 

REGULATOR 

AND 


400 -V) 

(380 TO 500'V) 

FILTER 






L 

^^^HELI POTS- 

j 







DRIVER 


DRIVER 







Fig. 29-04 Rectangular to polar 
coordinate resolver circuit 


Ground Position Indicator (GPI) 


29.07 


t 


N 



compass Spinner Resolver 



Different phase properties of this voltage are used^tp 
eliminate effect of undesired null at point B. 



Fig. 29-05 Sjrnchro system for coordination of azimuth data 


The range marker consists of a momentary oscilloscope intensification for 
all spinner azimuths and occurs simultaneously with the radar returns from objects 
on the ground at the same ground range as the reference point upon which the elec- 
tronic index is set. The range marker is controlled by a triangle solver containing 
circuits similar to those used in the GPI resolver to convert rectangular coordinates 
to polar coordinates. Two AC voltages differing in phase by 90®, and proportional 
to altitude and ground range respectively are combined to yield a resultant, the am- 
plitude of which is proportional to the slant range. This AC voltage is rectified, and 
the resulting DC voltage is used to control the delay introduced by the range marker 
circuit. 


29.08 

Ground Position Indicator (GPI) 


Bibliography 




Identification 

Classification 

Title 

Issued By 

S-19 

Confidential 

Ground Position Indicator for 
Radar Navigation and Bombing 

MIT Rad. Lab. 

Section 4 

Navigational 

Radar 

Secret 

UB. Radar Survey 
(pages 4-1 to 4-4) 

Div. 14 NDRC 


Miscellaneous Enemy Navigational Systems 


30.01 


This section contains brief descriptions of some of the lesser known German 
navigational systems. Much of the information has been obtained from prisoners of 
war, and hence is of questionable accuracy. 

Electra is a German navigational system of the azimuth type employing 
radio beacon stations each of which maintains a fixed multi- lobe pattern of adjacent 
dot and dash sectors. The transmission consists of unmodulated CW dots and dashes 
at a frequency between 290 and 480 kcps. The azimuth indication is of the aural 
dot-dash variety with a steady tone for the on- beam indication. The navigating craft 
carries DF equipment in order to resolve sector ambiguities in the beam pattern. 
The on- course beam width is about 0.3® giving a theoretical bearing accuracy of 
+ 0.1 5®. The maximum usable range is claimed to be of the order of 1 500 miles. The 
Electra system is also mentioned in the discussion of the German Sonne system in 
Section 17. The Electra field pattern is given in Figure 17-03. 

The German Benito system may be used to control both the range and azi- 
muth of an aircraft from a ground installation. The range to an aircraft is deter- 
mined at the ground station by a phase- shift method and the range inf ormation is 
relayed to the aircraft over the radio communications channel. The groimd- station 
carrier is amplitude modulated with a 3000 cps tone which is received by the air- 
craft and retransmitted on a different carrier-frequency. The range to the aircraft 
is obtained at the ground station by comparison of the phase of the modulation enve- 
lope of the received aircraft transmission with that of the original tone modulation 
of the carrier transmitted from the ground station. The fine range measurement 
contains a range ambiguity of some integral number of 50 kilometers. This ambi- 
guity is eliminated by a course range measurement using a tone modulation frequency 
of 300 cps. When used for the control of bombers, the azimuth information is ob- 
tained at a ground station by direction-finding on the reply signal of the range mea- 
suring channel. The azimuth information is then transmitted to the aircraft by cod- 
ed keying of the same transmission which is used for range measurement. 

Benito transmitters and receivers 
operate at frequencies in the 40- 50 mcps 
Imnd, with an average carrier power of 
about 0.8 kw. The maximum useful range 
is between 100 and 200 miles depending to 
a great extent upon the altitude of the air- 
craft. The accuracy of range measure- 
ment is reported to be about +50 yards 
with a skilled operator. 

Knickebein is a German navigation 
al system used primarily for blind bombing. 
It is an azimuth type of system employing 
fixed radio-beams with a rather complex 
arrangement of the dot and dash areas as 
shown in Figure 30-01. Details concern- 
ing the unused back portion of the beam 
pattern are not known. Either meter- 
indication or aural dot-and-dash signals 
are used for the on- beam indication, and no 
range information is provided. Two beams 
may be crossed over a desired target. The 
angular spread of the equisignal Knickebein 
beam is about 0.3 degree which is suitable 



Fig. 30-01 The Knickebein beam 


30.02 


Miscellaneous Enemy Navigational Systems 


for bombing a target the size of a town. 
Knickebein stations operate at a radio 
frequency between 30 and 33.4 mcps with 
an average power of about 1 kw. The max- 
imum usable range is claimed to be of 
the order of 250 miles, and the claimed 
bearing accuracy is about + 0.15^. 

Ruffian is a blind bombing system 
using a complex arrangement of radio beams 
each beam operating on a slightly different 
frequency in the region of 80 mcps with an 
average power of about .8 kw. The system 
utilizes both coarse and fine beams, the 
Fig. 30-02 Coarse Ruffian beam characteristics of which are illustrated in 

Figure^ 30-02 and 30-03 respectively. The 
equisignal zone between dot and dash areas is 4 wide in the coarse beam, and 0.1° 
wide in the fine beam. The fundamentals of the Ruffian system are illustrated in 
Figure 30-04. The bombing aircraft flies with constant speed toward A along a beam 
emanating from the beacon transmitter at T^. Beams from stations at T2 and T3 
intersect the aircraft course at points P and Q which are spaced 15 kilometers apart. 



PQ 

The height of the aircraft and the ratio are set into an automatic clock mechanism. 

Keys are pressed when the aircraft passes points P and ^ and the clock mechanism 
automatically releases the bombs at the correct point R, the location of which de- 
pends upon th€ time required for the aircraft to pass from P to Q. As actually used 
station T^ transmits a coarse beam in addition to a fine beam so that the pilot can 
fly the coarse beam during the early part of the flight in order to avoid fatigue. Sta- 
tion T 3 also transmits a coarse warning beam which the aircraft passes through short- 
ly before reaching point P. Two extra emergency transmitters are also available 
one near Tj and the other near T2 and T3 in case the regular transmitting stations 
should be either jammed or otherwise not functioning properly. Under ideal condi- 
tions the bombing accuracy is reported to be of the order of + 80 meters at the maxi- 
mum range of about 200 miles. 



Fig. 30-03 Fine Ruffian beams 


Hermine is a VHF German naviga- 
tional system in which no special electron- 
ic equipment other than an ordinary com- 
munications receiver is required aboard 
the navigating craft. It operates on a fre- 
quency between 30 and 33.3 mcps. At the 
ground station the Hermine rotating bea- 
con transmits a continuous tone upon 
which is superimposed a speaking clock 
which counts from 1 to 35, each figure re- 
presenting tens of degrees in azimuth 
angle. A beacon-identification code-name 
is spoken in place of the figure 0, and the 
entire cycle of events takes place in one 
minute. The continuous tone partially 
masks the voice modulation except in a 
small sector of about 15® within which 
the masking tone gradually falls to zero 
and rises again so that the voice modulation 
momentarily becomes more audible . Pre- 


Miscellaneous Enemy 'Navigational Systems 


30.03 


A 



Fig. 30-04 Elements of the Ruffian system 

sumably the field pattern of the masking signal is a rotating cardioid. The null in 
the masking tone moves uniformly through the 360® of azimuth and an observer can 
estimate the position of the null to within 3® to 5®. A navigational fix is obtained by 
taking bearings on two Hermine stations. Although its accuracy is not very great, 
the Hermine system has the advantage of great simplicity. 

Ruebezahl (or Egon ) is a German bomber- control navigational system in 


30.04 


Miscellaneous Enemy Navigational Systems 


which one aircraft is controlled by two Freya radars. The position of the aircraft 
is continuously plotted from Freya data, and course corrections are transmitted to 
the aircraft by either code or voice signals on a radio communication channel. 
Either aural or visual indications may be used. One Freya plots the course of the 
aircraft until it is quite near the target and then a more accurate Freya takes over 
for the bomb release instructions. In the Ruebezahl or Egon system, IFF responses 
from equipment in the aircraft greatly increase the range of the Freyas. The air- 
borne equipment operates on a frequency of 116 to 146 mcps with pulses of 2 to 3 
microseconds duration, and with a peak power of from 15 to 20 kw. The aircraft 
may be controlled in range to within ±100 yards and in bearing to better than± 1°. 


Bernhard- Bernhar dine is a German navigational system used by night fighters 
for the interception of an enemy bomber force. Bernhard is the groimd station and 
Bernhardine is the aircraft installation used with Bernhard. The ground station 
(Bernhard) is a very large rotating antenna array 34 meters in diameter and mak- 
ing two revolutions per minute . It operates on a wavelength of between 7 and 9 meters 
with a power output of the order of lkw.,and is essentially a ranging and direction- 
finding set. In the night-fighter aircraft the Bernhardine equipment gives a contin- 
uous indication of the bearing of the Bernhard station, and also gives the location, 
course, altitude, and approximate strength of the enemy bomber force which the 
night fighter is trying to intercept. All of this information is printed on a tape once 
per minute, the printing requiring 10 seconds. The printing on the tape appears as 
shown in Figure 30-05. In the case of Figure 30-06, the azimuth or bearing of the 
Bernhard ground station from the night fighter aircraft is 90° as indicated on the 
scale directly below the V-shaped notch in the printed lines. The bearing is accurate 




||||■|lll’lpMl|lllr|'lMl||lll|llM||||r]1||||||||||||||Mll| 

6X 7 8X 9 lOX I I I2X 


+40 KA 27 100 


Fig. 30-05 Typical Bernhardine printed tape presentation 


to within 0.5®. The figures appearing below the horizontal azimuth scale form a cod- 
ed message. The + sign indicates the start of the message. The figure 40 indicates 
the height in hundreds of meters of the leading enemy bombers in the formation 
being attacked. The letters KA indicate a coordinate grid position of the night-fight- 
er at the head of the attacking stream. The figure 27 indicates in tens of degrees 
the azimuth of the enemy bomber formation under attack, and the final figure 100 is 
an estimate of the number of bombers. 

Hyperbol (or Hyperbel) is the German copy of the British GEE system which 
is discussed in Section 11. 


Truhe is a German navigational system very similar to Hyperbel. 

Zyklpp (or Cyclop ) is a German navigation beacon similar to Knickebein. 
The radio transmission is in the 30-33.3 mcps band, and consists of intermittent 
CW with sine-wave modulation. The indication is of the dot-dash variety, either 
aural or visual, on a kicking- type meter. Zyklop is believed to be mobile Knickebein. 

is a German rotating radio- beacon with a figure- eight field pattern. A 
bearing on one beacon is obtained by measuring the time interval between a minimum 
signal and a fixed marker signal. Dora beacons operate in the 30-100 meter band 
with a power output of about 1.5 kw. The maximum usable range is of the order of 


Miscellaneous Enemy Navigational Systems 


30.05 


1000 kilometers. 

Erika (and New Erika) are beacons giving a beam pattern which makes detec- 
tion of an intended target extremely difficult. It is similar to Knickebein but there 
is no single beam upon which a bomber aircraft attacks a target, and hence, night 
fighters cannot attack by flying on the beam. It is based on the principle of a VHF 
(30-33 mcps) beam oscillating rapidly over a segment of about 60° - 90^. The beam 
has a different phase in different sections of the segment and the phase of the re- 
ceived signal is compared with that of a standard phase producer in the aircraft. 
Six dials automatically indicate zones, and fine zones do not have to be flown until 
just before dropping the bombs. Its main disadvantage is its vulnerability to jam- 
ming. 


Diskus is a German radar navigational and bombing system in which an air- 
craft flies a circular course at constant range from one ground station A and releases 
its bombs upon reaching the proper range from a second ground station B. A pulse 
transmitter is located in the aircraft, and responder beacons are located at the 
ground stations. It is similar to the Micro-H system as used to fly a cat- mouse 
course (See Section 5). 

Schwanboje beacons are German floating target beacons operating on fre- 
quencies between 42.1 and 47.9 mcps. A navigating craft may home on the target 
beacon by direction-finding on its radiated signal. 

The German Nachtfee system is a method for transmitting fighter direction 
commands over the beam of the controlling radar by space (phase) modulation of the 
Freya pulses within a repetition cycle. In order that a number of fighters may be 
controlled from a single Freya ground station, the airborne equipment is built in 
10 different models with crystal controlled repetition rate oscillators operating at 
frequencies grouped around 1 5 kcps . Fighter direction commands are indicated by 
the angular position of pips appearing on a circular sweep CRO trace. 


30.06 


Miscellaneous Enemy Navigational Systems 


Bibliography 


Identification 

Classification 

Title 

Issued by 


Secret 

Notes for Pilots, Observers, 
and special W/T operators on 
enemy radio aids to navigation 
and accurate bombing under 
blind conditions. 

Headquarters of 
North 80 Wing, 
RADLETT 

WA- 2266- 12 
Loga Z 349 

Secret 

German bombing and Naviga- 
tional aids. 

ETOUSA, OC Sig O 
RCM division 

No. 357/1945 
Loga L-2641 
JEIA 10784 

Secret 

Radio and radar equipment 
in the Luftwaffe- II 

A.D.I.(K) 

Loga L-2520 
JEIA 10555 

Secret 

BHF Rundschreiben Nos. 3, 

4, 5; 9 July 1945 

OC Sig O Headquar 
ters U. S. Forces 
European Theatre 

Loga F 1583 

Secret 

German "Nachtfee" system 
of control of night fighters 

UjS. Naval Techni- 
cal Mission in 
Europe 

Loga L-873 
JEIA 7887 

Secret 

Extract from USSAFE Air 
Intelligence Summary 66 


Report No. 33 
20.4.45 

Secret 

Air Scientific Intelligence 

Air Ministry, 

A J).I. (Science) 


Comparisons, Conclusions and Recommendations 


31.01 


Comparison of Navigational Systems 

Any universal comparison of the various electronic navigation systems here 
described must necessarily be inadequate because of their widely differing objectives . 
Thus a short-range high-precision system such as Shoran may not properly be com- 
pared with a long-range system such as Sonne. The following comments are how- 
ever of a general nature, and are sufficiently fundamental to warrant the attention 
of anyone interested in electronic navigation, 

1. Pulse systems, in which the received signals are displayed on a CRO, are not 
nearly so susceptible to meaconing, or to hidden errors produced by varying pro- 
pagation conditions, as are continuous -wave systems. The reason for this is 
two-fold. Firstly, any irregularity in received pulse signals (such as spurious 
pulses, distortion of pulse shape due to plural-path transmission) is visible to 
the operator on the CRO trace. (Cathode- ray indicators have produced misgiv- 
ings among uninitiated personnel, due to a natural distrust of glassware and hid- 
den wires, but they have an enormous versatility compared with meters, audible 
indicators or automatic- control systems.) Secondly, a pulse system offers the 
possibility of separating ground- wave and sky-wave returns. As pointed out in 
connection with SS Loran, the minimum delay between sky-wave and ground-wave 
signals is of the order of 65 microseconds at 2 mcps (possibly lower at LF Loran 
frequencies), so that if the rise-time of the pulses (as seen by the navigator) is 
short enough, the leading edge of the pulse represents ground- wave transmission 
only and is therefore dependable. In continuous-wave systems however, the trans- 
mission is not broken up into discrete pulses so that ground- wave and sky-wave 
returns cannot be separated. 

2. Pulse systems require a larger band-width than CW systems and are therefore 
more easily accomodated at high than at low frequencies. However, the use of 
high frequencies would impose a limitation on ground- wave range, and the ten- 
dency with long-range Systems of any kind is therefore towards the use of lower 
frequencies. With pulse transmission, this leads to two difficulties; wide-band 
transmission produces considerable adjacent- channel interference, and the attain- 
ment of the required band- width in the radiating system is difficult even with 
very high antennas. The total spectrum allotment required for a pulse system 
may be kept down to a reasonable figure by "stacking” a number of transmitters 
(using different pulse repetition rates) on the same frequency as is done in the 
case of Loran. 

3. Provided that sufficient ingenuity is exercised and that there are no restrictions 
on weight and space, any* given system can be made to present its final indication 
in any desired form. Automatic piloting, applied to either aircraft or guided 
missiles, could presumably be realized with Loran transmissions. At the other 
end of the scale, the Sonne system probably requires less additional equipment 
on the craft than any other. 

4. The comparative value of radar as a navigational aid 

(a) Airborne Radar. Any general navigational system must make use of a chart. 
In the case of non- radar systems, the chart usually exhibits a number of lines of 
position, each corresponding to a particular indication given by the craft equip- 
ment. These lines of position may be of a special nature, as in the case of Loran 
hyperbolae, or they may be merely spherical coordinates of latitude and longi- 
tude. In either case, the navigator determines his position by identifying lines of 
position on the chart. 

A Radar system giving a PPI presentation is different in that the navigator 
now has two charts, one of which is his PPI presentation. Superposition of the 
two (either actually, as in NALOC, or mentally as in ordinary use of a PPI pic- 


31.02 


Comparisons, Conclusions and Recommendations 


ture for navigation) gives the navigator' s position. 

The Radar method has the advantages of displaying the positions of other 
craft (and evenof storm centers), and of requiring no ground station transmission 
over areas where natural Radar landmarks exist: it can be used in areas where 
Loran coverage is not provided or is unreliable. Radar, being a versatile tech- 
nique, presents opportunities for other specialized uses: for example, collision 
prevention, Radar mapping, communication by pulse-time modulation, retrans- 
mission of an airborne PPI display to a ground or shipborne information center. 
Disadvantages are that oversea navigation IS not possible at distances from Radar 
landmarks greater than the Radar range, that over land a number of landmarks 
having a particular disposition is necessary for certain identification of position, 
that there is distortion in the PPI picture at close range due to slant range data 
being presented instead of ground range data (unless a h^erbolic sweep is used), 
and the additional weight, complexity and cost of the equipment as compared with 
(say) a Loran receiver and indicator. 

The use of beacons or corner reflectors with airborne radar provides a 
system of identification of a particular course, airport or natural hazard. These 
are problems of restricted or special navigation and as such are very elegantly 
handled by radar means. 

Unless beacons are used, the accuracy of a Radar fix may not be equal 
to that of a good Loran fix, but it would appear to be more constant over a large 
area if available at all, and is probably adequate for purposes of general naviga- 
tion. 

(b) Ground Radar. In situations where the emphasis is on ground control of air 
traffic rather than on the presentation of individual fixes to pilots of aircraft, 
ground radar may well become the accepted solution to the control problem. Any 
complete control system must include some communications link so that coor- 
dinated information and specific instructions can be transmitted to the pilot of 
the craft. Such a system is expensive to install and maintain, and would only be 
justifiedat airports, control points, and possibly at major hazards. Several close- 
ly coordinated schemes have been proposed, such as the RCA system and the 
Federal traffic -control system. A disadvantage is that such systems must be 
used by all of the traffic if true ground control and collision prevention are to be 
obtained with zero visibility, requiring certain equipment to be compulsory on 
all craft. Such a coordinated system requires careful design if the problem of 
identification is to be fully solved. Unless identification is provided for, the 
function of control cannot be exercised fully on individual craft. Such systems 
may be designed so that the craft equipment may also be used for blind approach 
and possibly blind landing techniques. Such extensions are essential if all-weath- 
er commercial air traffic is to become a reality. 

5. Regarding accuracy and precision of position-line determination, the following 
general statements may be made: 

(a) Range- measuring systems have the same position-line precision at all ranges, 
since the separation of the circular position lines in miles per microsecond 
is constant. This statement neglects errors in crystal control, which are 
usually very small. Hyperbolic and azimuthal systems give a precision in 
line of position which varies inversely as the range, at distances greater 
than about five times the base line. 

(b) Azimuthal precision is uniform at all azimuths with systems using rotatable 
antennas , such as radar systems. It is not uniform with hyperbolic systems, 
being greatest along the perpendicular bisector of the base-line. 

(c) Continuous- wave systems such as Decca offer great accuracy at short ranges 
if the ambiguity involved can be tolerated. This great accuracy arises from 


Comparisons, Conclusions and Recommendations 


31.03 


the precision resulting from phase measurement at radio frequency. Pro- 
portionately good accuracy at great ranges is claimed if a long base-line is 
used. However, along base-line means that the system can be of only limit- 
ed range for reliable results, because the use of signals containing any 
appreciable amount of sky-wave return, or consisting solely of sky waves, 
is ruled out. Consider for example a base-line of 300 miles. Using sky 
waves for phase comparison, the distance between the points where ionospheric 
reflection takes place for master and slave transmissions to the craft will be 
of the order of 1 50 miles. Since there is no reason to expect that ionospheric 
conditions at these two widely- separated points will vary in a sufficiently 
similar way over a period of time, the phase difference measured at the 
craft will show wide variations with corresponding uncertainty in the line of 
position obtained. 

A .similar argument applies to signals composed of a mixture of 
ground- and sky-wave returns. Variations in the phase and amplitude of the 
sky-wave component will produce variations in the phase of the resultant. 
Assuming for the purpose of illustration a Decca system working on a fre- 
quency of about 200 kcps, a complete reversal in phase of the sky-wave sig- 
nal (180^ phase change) represents only 2.5 microseconds change in time 
of travel. Studies of the variations in sky-wave delay which have been made 
in connection with Loran pulse transmissions show that short-period varia- 
tions of several times this amount may easily occur and are not at present 
predictable. In the case of mixed sky- and ground- wave returns, the maxi- 
mum difference in phase (in degrees) between the resultant signal and the 
ground-wave signal is the angle whose sine is the ratio of sky-wave ampli- 
tude to ground^ wave amplitude. This maximum difference will be encounter- 
ed when the phase of the sky-wave is approximately in quadrature with that 
of the ground- wave (if the sky-wave amplitude is not too large a fraction of 
the ground- wave amplitude). Thus, if the amplitude of the sky wave is as 
much as one- sixth of that of the ground wave, a maximum phase shift of about 
+ 10^ in the resultant signal will be encountered as the phase of the sky-wave 
component fluctuates about its average value, and changes in amplitude will 
also occur. Since the accuracy of the Decca system depends on phasemeters 
which are sensitive to a 2® phase change, it will be seen that the amount of 
sky-wave return which can be tolerated is very small. Thus a continuous- 
wave system depending on phase comparison is essentially limited to the 
area within which pure ground- wave reception is obtainable. 

The extent to which the above considerations may be modified by using 
a lower frequency cannot be accurately foreseen, since data onpropagation at 
such frequencies is lacking at the present time. 

(d) Where extreme range and good precision are both desired, LF Loran (with 
cycle matching) appears to be an attractive possibility. 

6. Sky-Wave errors. In addition to the fluctuations in position-line reading mention- 
ed above in 5(c), which will be evident to the operator and therefore not dangerous 
(even though they limit the useful range of a system), large and relatively consis- 
tent errors in line of position may result from sky-wave propagation in systems 
depending on radiation patterns (radio "ranges”, Sonne, Federal long-range sys- 
tem). Consider as an example the Federal system, in which a sequence of two 
pairs of directional patterns is radiated by changing the relative phasing of the 
currents in two antennas spaced one- half wavelength apart. At the receiver, the 
stronger signal from each of the two pairs is attenuated to give output signal 
ratios of unity, and a line of position is then obtained from the attenuator settings. 
(It may be remarked in passing that transmitter and receiver band-widths must 
be considerably greater than the values quoted to reproduce adequately the modu- 
lation envelope shown.) Now if sky waves are used, the difference between the 


31.04 


Comparisons, Conclusions and Recommendations 


transmission paths from the two antennas to the receiving point at some given 
azimuth will no longer be the same as it would have been for ground-wave pro- 
pagation at the same azimuth, being a function of the angle at which the radiation 
leaves the ground station to reach the point of ionospheric reflection. Thus the 
position- lines will be skewed by an amount which depends on the transmitter- re- 
ceiver distance and also on the azimuth of the receiver with respect to the trans- 
mitter. If this were all, the necessary sky-wave corrections would be calculable 
and could be applied by means of appropriate markings on the charts used. Vari- 
ations in effective point of reflection, and in the relative magnitudes of returns 
from different ionospheric layers, will still produce a range of uncertainty in 
the readings. However, a much more serious consideration is that ground waves 
and sky waves are not separated, so that the percentage of sky-wave return pre- 
sent and the magnitude of the correction to be applied are unknown and inconstant 
factors. This is the fundamental argument which rules out the long-range use of 
such systems and will continue to do so until a great deal more is known about 
ionospheric propagation in general. 

7. Range- measuring systems usually require transmission from the craft (an ex- 
ception would be in the case of a ground- radar display which is communicated 
to the craft by audio or video signals). Range- measuring systems are saturable: 
hyperbolic and azimuthal systems are not. 

8. It may be said that the subject of frequency and band-width allocations is very 
intimately connected with the development of long-range navigation, and such 
allocations will be a deciding factor in the effectiveness of any systems which 
may be developed. In this connection, what is important is the bandwidth required 
for a complete navigation system. With most systems, received signals are be- 
low the local noise level beyond a radius of about 3000 miles from the transmit- 
ter, so that channels assignedfor (say) North Atlantic coverage may be used again 
in the Pacific or over Asia. Thus a pulse system, such as Loran, may cover the 
North Atlantic with a 50-70 kcps tendwidth allotment since the use of different 
pulse repetition rates allows the stacking of as many as 16 stations at the same 
frequency. A continuous -wave system such as Sonne, requiring about the same 
number of stations to give the same coverage, will need about the same bandwidth 
allotment for the complete system. The apparent advantage claimed for narrow- 
band systems is thus largely illusory. In addition, a Decca system to give the 
same coverage has the further complication of requiring a number of frequencies 
so spaced that the necessary numerical relations exist between them. 

A chart showing the frequency- distribution of the various systems is given on page 
31.05, and a table summarizing their principal characteristics appears on pages 
31 .06 and 31.07. Tabulation is useful only in so far as it presents unambiguous data, 
and for this reason the table of principal characteristics does not attempt to sum- 
marize the facts concerning such matters as siting requirements for transmitters. 
Four aspects of siting may be mentioned: 

(a) The degree of accuracy required in the triangulation of proposed sites varies 
with the accuracy of the system. For Shoran, the triangulation must be extremely 
refined if the full capabilities of the system are to be realized. 

(b) The size and expense of the ground system required is greater at low frequencies. 

(c) The elevation of the transmitter site above the local horizon affects primarily 
the range attainable with ground waves. 

(d) The flatness of the site, and the absence of non- uniformities, are factors of 
prime concern in systems such as radio ranges, Sonne and the Federal long-range 
system which depend on radiation patterns . Non- uniformities in the ground may atten- 
uate the radiation in some directions, thereby modifying the theoretical radiation 
pattern. 


Comparisons, Conclusions and Recommendations 


31.05 


NQ 

S 


IB‘ 


IB- 

S 

IB— I 


S- 


IB‘ 


IB* 


s- 


10 30,000 

kcps meters 


20 


50 


100 

kcps 


200 


500 


imcps 3oom 


lomcps 3om 


20 


50 


^ DECCA 


FEDERAL 
LONG RANGE 


L.F. LORAN 

AN RANGE 
AND CAA LF 
OMNI-RANGE 


SONNE 


POPI 


ADF AND 
> BENDIX 


SS LORAN 
STANDARD LORAN 


GEE 


No useful 
sky woves 

above 

3omcps 


100 3m 

mcps 


BLf 


N- 


B 


N 

Nc 


Ntl 


nc 


f 


100 

mcps 


3m 


200 


500 


1000 30cm 
mcps 


2000 


]S-BAND {^1^534 


5000 


10,000 3cm 
mcps 


20,000 


50,000 


100,000 3 mm 


200,000 


500,000 


CAA VHF OMNI- RANGE 

CANADIAN 

AN/APN34 

SHORAN 


X-BAND 


MICRO-H 

HgX 


- K-BAND 


N= Navigational Aids 

B= Broadcast 

S= Standard frequency 
broadcast 

IB= International 
broadcast 



31.06 Comparisons, Conclusions and Recommendations 



Type of 
System 

Maximum useful range 
(statute miles)* 

Uncertainty in line of position 

Presentation 
to navigator 

Special skills 
(craft) 

Craft 

Equipment 

Min. Theoretical 

Ambiguities 

Oboe 

Range 

Radar line-of-sight* 

+ 25 yards 

none 

Aural 

some 

training 

300 lbs. 
specialized 

Shoran 

Range 

180 (at 12,000 ft.) 

+ 50 ft. 

not less 
than 100 
mi. apart 

CRO 

Trained 

Operator 

232 lbs. 
specialized 

Micro-H 

Range 

Radar line-of-sight+ 

i 50 yards 

none 

PPI 

Trained 

Operator 

15 lbs. plus 

H 2 X equipment 

ARL Intermittent 
Phase-Comparison 

Range 

36 

not known 

36-mile 

intervals 

Meter 

very little 

specialized 

ARL One-Shot 

Range 

100 

not known 

none 

Veeder 

Counter 

very little 

specialized 

Canadian 

Range 

100 

± 1 mile 

none 

Meter 

very little 

specialized 

GE Random 
Interrogation 

Range 

100 

not known 

none 

Veeder 

Counter 

very little 

specialized 

GE Time- 
Rationing 

Range 

100 

not known 

none 

Meter 

very little 

specialized 

Gee 

Hyperbolic 

400 (at 30,000 ft.) 

(see page 11.01) 

± 0.062 mi. 

= 327 feet 

none 

CRO 

Trained 

Operator 

92 lbs. 
specialized 

Loran (standard) 

Hyperbolic 

Day: 850(G) 

11600(S) 

± 0.093 ml. 

= 492 feet 

none 

CRO 

Trained 

Operator 

specialized 

equipment 

70 lbs. 
with small 
frequency 
converter 
if used 
for LF 

Loran 

Loran (SS) 

Hyperbolic 

1600 (S) (night only 
no day-time trans- 
mission) 

± 0.093 mi. 

= 492 feet 

none 

CRO 

Trained 

Operator 

Loran (LF) 

Hinperbolic 

1500 to 2000 (?) 

♦2000 ft. 

♦50 ft. 

none 

CRO 

Trained 

Operator 

Decca 

Hyperbolic 

1500 (?) (Limited 
to ground wave) 

140 feet (?) 

multiple 

Meters 

very little 

specialized 

POPI 

Hyperbolic 

(Limited to ground 

Wave if gross errors 
avoided) 

0.636*^ azimuth 

none 

Meters 

very little 

standard 
rec. with 

POPI ind. 

AN "Range" 

Azimuth 

200 

±50 

two or four 

Aural 

very little 

low-freq. 

receiver 

Automatic 

Direction 

Finding 

Azimuth 

200 to 400 

±3° 

none 

Pointer 
and scale 

none 

specialized 

Sonne 

Azimuth 

Day: 1000 

Night: 2000(S) 

± 1/6° 

multiple 
(solve by 

DR or DF) 

Aural 

very little 

low-freq. 
receiver 
with BFO 

Bendix 

Azimuth 

200 to 400 

±3° 

none 

Automatic 
plotting 
on chart 

very little 

2 ADF's, 
compass, 
computer 

CAA VHF 

Omni- "Range" 

Azimuth 

100 

±3° 

none 

Meter and 

Dial 

very little 

specialized 

CAA LF 

Omni- "Range" 

Azimuth 

(serious sky-wave 
errors probable) 


none 

Meter and 

Dial 

very little 

specialized 

Federal 

Long-Range 

Azimuth 

1500 (?) (serious sky- 
wave errors probable) 

±0.2 to 1.6® 

two: 

solved by 

DF 

Meter and 

Dials 

very little 

specialized 

Airborne Radar 
(HzX) 

Range and 
Azimuth 

250 (beacon) 

90 (search) 

±1-3° azimuth 
± 50 yds. range 

none 

PPI 

Trained 

Operator 

370 lbs. 
specialized 

Ground 

Radar 

SCR 584 

Range and 
Azimoth 

40 

± 0.034° ± 15 yds. 

none 

via commimi- 
cation channel 
from ground 

none 

Communica- 
tion, beacon 

MEW 

250 

± 1 mile 

none 

AN/APN 34 

Range, 

Azimuth, 

Track 

50 to 100 

± 2°± 1/2 ml. 

none 

Meters 

very little 

specialized 

Federal 

Traffic- 

Control 

Range and 
Azimuth 

50 to 100 

? 

none 

CRO 

very little 

specialized 

Map-PPI 

Naloc 

Range and 
Azimuth 

10 to 100 

±1-3®, 

± 200 yards 
approx. 

none 

PPI 

SkiUed 

Operator 

Radar set, 
projection 
equipment 

RCA 

Range and 
Azimuth 

200 (line-of-sight) 

equal to that 
of ground Radar 
used 

identi- 
fication (7) 

Television 

image 

Interpretation 
of PPI 

television 
rec., beacon 
transponder 

Sperry 

Range and 
Azimuth 

100 azimuth 

24 range 

± 30 azimuth 
± 400 ft. range 

Range over 

24 mi. 

Meters 

very little 

specialized 

AN/APA 44 

GPI 

Range and 
Azimuth 

1000 mi. from last 
reference point 

± 3 % of range 
from reference 
point 

reference 
Radar echo 
recognition 

PPI and 
dials 

Skilled 

Operator 

175 lbs. 
plus Radar 


* figures assume average 
noise level 


(2) 

' 'envelope matching 
'^^cycle matching 


t tentative figures 


IS' 


, . = ground wave 
,S) = sky wave 


t approximately 250 miles at 30,000 ft. altitude 


Comparisons, Conclusions and Recommendations 31.07 



Frequency 

Band- 

width 

Type of 
transmission 

Ground wave and 
sky wave separated 

Meaconing 

Recognizable 

Status 

Saturable 

Transmission 
required 
from Craft 

Instantaneous 
fix with single 
equipment 


S-band 

8 mcps 

Pulse (1-3 ps) 

(No sky wave) 

- 

Opera- 

tional 

Yes: one 

craft at 
a time 

Yes 

Release point 
indicated 


210 - 320 
mcps 

4-5 

mcps 

Pulse 
(0.5 ;js) 

(No sky wave) 

Yes 

Opera- 

tional 

Yp«? 

(20 craft) 

Yes 

Yes 


X-band 

2.5 

mcps 

Pulse 

(2ps and 0.5ps) 

(No sky wave) 

Yes 

Opera- 

tiotial 

Yes 

(50-100 

craft) 

Yes 

Yes 


VHF or 
above 

6 kcps 

Intermittent 
Continuous Wave 

(No sky wave) 

No 

Develop- 

ment 

Yes 

Yes 

No 


VHF or 
above 

not 

known 

Pulse 

(No sky wave) 

No 

Develop- 

ment 

Yes 

Yes 

No 


202 ■ 220 
mcps 

3-4 

mcps 

Pulse 

(No sky wave) 

No 

Develop- 

ment 

$ 

Yes 

Yes 

No 


VHF or 
above 

not 

known 

Pulse 

(No sky wave) 

No 

Proposed 

Yes 

Yes 

No 


VHF or 
above 

not 

known 

Pulse 

(No sky wave) 

No 

Proposed 

Yes 

Yes 

No 


20 - 85 
mcps 

1 mcps 

Pulse 
(2 - lOps) 

(No sky wave) 

Yes 

Opera- 

tional 

No 

No 

Yes 


1700-2000 

kcps 

50-70 

kcps 

Pulse (40 /Js) 

Yes 

Yes 

Opera- 

tional 

No 

No 

No 

1700-2000 

kcps 

50-70 

kcps 

Pulse (40ps) 

Yes 

Yes 

Opera- 

tional 

No 

No 

No 

180 kcps 

10(?) 

kcps 

Pulse (300/Js) 

Yes, if front edge 
of pulse used 

Yes 

Trials 

No 

No 

No 


20 - 200 
kcps 

three 

single 

freq. 

Continuous Wave 

No 

No 

Trials 

No 

No 

Yes 


750 kcps 
(trials) 

1 kcps 
or less 

Continuous Wave, 
slow keying 

No 

No 

Small- 

scale 

trials 

No 

No 

No 


200 - 400 
kcps 

3 kcps 

Continuous Wave, 
slow keying 

No 

No 

Opera- 

tional 

No 

No 

No 


100-1600 

kcps 

single 

freq. 

Continuous Wave, 
no modulation 
necessary 

No 

No 

Opera- 

tional 

No 

No 

No 


250-500 

kcps 

1 kcps 
or less 

Continuous Wave, 
slow keying 

No 

No 

Opera- 

tional 

No 

No 

No 


100-1600 

kcps 

two 

single 

freqs. 

Continuous Wave, 
no modulation 
necessary 

No 

No 

Proposed 

No 

No 

Yes 


127 mcps 

24 kcps 

double 

modulation 

(No sky wave) 

No 

Trials 

No 

No 

No 


200 - 400 
kcps 

3 kcps 

double 

modulation 

No 

No 

Develop- 

ment 

No 

No 

No 


70 - 76 
kcps 

70 cps 
(?) 

Continuous Wave, 
switched radiation 
patterns 

No 

No 

Proposed 

No 

No 

No 


X-band 

2.5 

mcps 

Pulse 

(0.5jus and 2ps) 

(No sky wave) 

Yes 

Opera- 

tional 

Yes 

Yes 

Yes 


S-band 

3-4 

mcps 

Pulse 
(1 jus) 

(No sky wave) 

- 

Opera- 

tional 

Yes 

No (unless 
beacons 
are used) 

Yes 


210 mcps 
approx. 

3-4 

mcps 

Pulse 

(No sky wave) 

No 

Experi- 

mental 

Yes 

Yes 

Yes 


7 

9 

Pulse 

(No sky wave) 

Yes 

Proposed 

Yes 

Yes 

Yes 


S-,X- 
or K- 
band 

that of 
Radar 

Pulse 

(No sky wave) 

Yes 

Opera- 

tional 

No 

Yes 

Yes 


S- or X- 
band 

that of 
Radar 

Pulse, television 
signal 

(No sky wave) 

- 

Proposed 

Yes 

Yes 

Yes 


C-band 

250 

kcps 

Modulated and 
keyed 

(No sky wave) 

- 

Proposed 

Yes 

Yes 

Yes 


X- or K- 
band ^ 

2.5 

mcps 

X-tond] 

Pulse 

(No sky wave) 

- 

Opera- 

tional 

No 

Yes 

Yes 


Note: Transmission over sea-water is assumed where the ranges 
quoted involve ground- wave transmission 


Transmitter siting requirements: see separate 
note in Section 31. 



31.08 Comparisons, Conclusions and Recommendations 

Coordination of navigation requirements 

In view of the number of electronic navigation systems which are or have been 
proposed, developed or operational, and considering the increasing importance of 
such systems in both military and civilian applications, it is evidently desirable to 
evolve a standard system which will meet the largest number of requirements with 
the maximum of reliability and the minimum of cost and complexity. It is therefore 
of interest to examine the requirements of a navigation system, and to attempt to in- 
dicate the extent to which a single airborne equipment might meet them. 

A general navigation system must provide indication of position while allow- 
ing the pilot free choice of course. Restricted navigation systems, which define a 
number of definite courses, have been successfully used in commercial point-to- 
point scheduled flying; but both military and civil interests require the fullest de- 
velopment of truly general systems. Any such system may of course be used in 
the more limited way: homing along a Loran hyperbola is a well accepted technique. 

Since the ability to navigate under poor visibility conditions is one olthe 
prime features of electronic systems, and since the lack of adequate blind approach 
and blind landing systems constitutes the chief obstacle to fully reliable air travel, 
it is assumed for purposes of this discussion that no land objects are optically yis- 
ible to the navigator. 

The ideal airborne electronic system must perform the following functions: 

(a) long-range general navigation 

(b) traffic control in the vicinity of airports 

(c) blind approach 

(d) blind landing 

The distances or time intervals to be measured for each function are not the same: 
they decrease in the order given. Position determination within two or three miles 
is normally sufficient for long-range navigation: for blind landing the accuracy re- 
quired is of the order of feet, corresponding to time intervals of the order of 0.001 
microsecond. 

For long-range navigation, ground- wave attenuation indicates the use of a 
low frequency, and the uncertainties inherent in the use of skywaves call for a pulse 
technique. Since the accuracy required is not of the highest order, long pulses with 
relatively long rise-times may be permitted. 

As the range of operation becomes shorter and the required accuracy higher, 
the use of higher frequencies becomes desirable since advantage may then be taken 
of directional transmission and since the shorter pulses required for the increased 
accuracy are more readily usable at high radio frequency. 

At very short ranges and where very high accuracy is required, the contin- 
uous-wave phase- matching technique might be considered. However, even if time 
intervals as short as 0.01 microsecond were to be measured by such means, the 
accuracy of distance measurement would still be barely sufficient for the blind- 
landing function. The limiting factor here being the high speed of propagation of a 
radio wave, it is natural to consider the use of sonic or supersonic methods for 
localizing a craft within such narrow limits. Since a discussion of blind- landing 
techniques is specifically excluded from this report, this subject will not be pursued. 

Radar techniques are the obvious solution to the control and blind approach 
problem: LF Loran is attractive as the primary long-range system. The design 
of a suitable ground-based short-range system using radar is largely a matter of 
deciding upon standardized control locations, identification methods and the prin- 
ciple of communication to be used. Two such coordinated systems have been des- 


Comparisons, Conclusions and Recommendations 


31.09 


cribed: RCA proposes to televise the information from ground to craft while Fed- 
eral would retransmit the video PPI signal directly. 

A general long-range navigation system over sea is entirely feasible, using 
Loran or Sonne. Over land, however , the number of ground stations required appears 
to be impractical owing to the reduced ground- wave range. The use of airborne 
radar with a combination of natural landmarks, beacons and corner reflectors is an 
alternative possibility although not too attractive. If any extended use of sky-wave 
transmission is to be made, further investigation into the properties of the ionosphere 
is very necessary. 

From what has been said, it is evident that a single equipment to meet all 
four requirements is not practical. It appears that the problems of long and short 
range navigation require fundamentally different techniques for their solution. 

The fact that a general over- land navigation system using LF Loran would 
require a very large number of ground stations may be seen by considering the pro- 
blem of an all-weather New York- Chungking air route by a great-circle path. Such 
a path passes close to the North Pole (see Fig. 31-01), and this route is not neces- 
sarily advocated as an economically workable proposition: it is chosen merely as 
an illustration for navigational discussion. 

The problem of providing polar Loran coverage is not easily solved if sta- 
tions are to be located in even semi-accessible areas. The distances are too great 
for a quadrilateral of the SS Loran type. The only solution appears to be a ring 
type of network, with a master station A controlling slaves and B 2 , which in turn 
control "sub-slaves” Ci, C2, etc. Such a chain lends itself to extended coverage 
and to the provision of multiple-line fixes, but the number of sub-stages of control 
would be limited by the accuracy of synchronization attainable. Double -pulsing of 
certain slave stations would be possible, but since a number of stations would have 
to use the same repetition rate, pulse identification (by means of double pulses or 
pulse- blinking) would have to be provided. 

Assuming for purposes of discussion that in polar regions synchronization 
would be obtainable at 1000 miles, and reception sufficiently reliable for position- 
line determination out to 2000 miles from any one station, it can be seen from Fig. 
31-01 that polar coverage may be provided by such a chain if suitable locations can 
be arranged and if difficulties of supply and maintenance can be overcome. Such a 
polar chain constitutes a separate unit, giving navigational facilities for all polar 
routes. The advantages of combining meteorological stations with navigational 
radio stations are apparent. Considerable data on ground- wave and sky-wave trans- 
mission in polar regions would be needed if such a scheme were to reach the stage 
of practical planning. 

Considering now the problems of navigation over the large land- masses 
adjacent to the polar regions, one must of course assume that the location, control 
and maintenance of Loran stations on any desirable territory by an international 
authority is realizable. Further, since any particular station must clearly be so 
situated as to be part of a coordinated system giving coverage for a number of re- 
gions or proposed air-routes, the locations shown in Fig. 31-01 probably do not 
represent a desirable solution to the general problem. Supposing the locations of 
the stations E , F, G, H, I and J (southern Greenland, Belle Isle, Nantucket, Charles- 
ton, Milwaukee, Port Nelson) to be so selected that coverage is given not only over 
the New York- Chungking polar route but also over the western Atlantic on one side, 
and the central United States on the other, it is seen that no less than six stations 
are required for this coverage alone, if the figures previously given for base-line 


31.10 


Comparisons, Conclusions and Recommendations 



Comparisons, Conclusions and Recommendations 


31.11 


and for range are retained. If sky waves are to be used, these figures might be 
realized over land, but difficulties would then be encountered with the cycle- match= 
ing technique. If ground waves only are to be relied upon, the number of stations 
required for this coverage might be more than doubled. 

Turning now to the problem of navigation over north-eastern and eastern 
Asia, the difficulties are similar but greater. To the east, there is a temptation to 
follow the coastline (stations K, L, M, N, O and P)^ but if this is done coverage over 
the Chungking route would probably depend on sky waves. Stations Q, R andS do not 
appear to fit in very well with other possible Asiatic air-routes, and the present 
state of development of this territory is certainly not such as to justify the outlay on 
a general navigational system to cover this region. 

The above discussion is only too inadequate, but perhaps enough has been 
said to illustrate the difficulties to be encountered in planning any general navigation- 
al coverage over land masses. If Standard Loran techniques and frequencies are 
considered, any such scheme is highly impractical if not impossible. Further re- 
search on propagation at LF Loran frequencies is required to determine whether or 
not such service could be realized using LF Loran. 

Recommendations for further research 

1. Basic research is required to determine all relevant information regard- 
ing the properties of the ionosphere, with particular attention to the lower fringe of 
the E-layer . For example, data on reflection coefficients and on the size, density 
and motion of ionic clouds would be of fundamental interest. Present ionospheric 
observations should be expanded to meet such needs. 

2. The research indicated above should be accompanied by a basic study of 
actual propagation paths, including studies of the plane of arrival of reflected signals, 
the nature of their polarization, variability in time of arrival and in signal strength, 
and modification of pulse shapes due to ionospheric transmission. A comparison 
should also be made of signals received simultaneously at receivers spaced a few 
hundred yards apart. Basic research is also required on ground- wave transmission 
over sea and over land, and also on at least one sky-wave transmission path such 
that detailed ionospheric data are obtainable at the mid-point of the single-hop path. 

3. A basic study should be made of the ideal specifications for pulse mod- 
ulation, and of the practical means for approximating this ideal, with special atten- 
tion to the low-frequency end of the spectrum. 

4. Concurrently with the basic studies outlined above, it would be desirable 
to continue operational tests on LF Loran and on Sonne, while employing standard 
Loran (for the present) for the relatively long-distance overseas navigation. 


A - Grant Land 
Bl - Victoria Island 
B2 - Spitzbergen 
Cl - Pt. Barrow 
C2 - Taimir Peninsula 
Di - Pokhodsk 
E - Greenland 


F - Belle Isle 
G - Nantucket 
H - Charleston 
I - Milwaukee 
J - Port Nelson 
K - Kamchatka 
L - Hokkaido 


M - Korea 
N - Formosa 
O - Hainan 
P - Rangoon 
Q - Lhasa 
R - Barkol 
S - Krasnoyarsk 


The dashed circles indicate approximately areas within 1000 miles and 2000 miles 
of station A. Such areas are not accurately represented by circles, since the pro- 
jection used distorts east-west distances. The distortion is however not serious in 
polar regions. 




Appendix A 


32.01 


A short glossary of terms used in this report 

Accuracy refers to the degree of concordance between a given measurement and the 
true value (which is assumed known). 

Azimuth angle or "Azimuth” is an angle measured in the horizontal plane. True 
bearing is an azimuth angle measured east (clockwise as seen from above) from 
true north. 

Craft is here used to designate surface vessels, aircraft, land vehicles, and guided 
missiles. 

A fix is defined as the point determined by the intersection of two or more lines of 
position. 

An interrogator is a pulse transmitter used to emit a signal which elicits an auto- 
matic response from a transponder. 

An inversion layer is an atmospheric layer in which the vertical temperature gra- 
dient, normally negative, becomes positive. 

A line of position is a line such that some point on it is the instantaneous position 
of the navigated craft. This line may or may not lie on the surface of the earth. 

Meaconing is the act of falsifying by radio means, the indications given by enemy 
radio navigation systems. The object of meaconing is to mislead enemy navi- 
gators by causing a false indication of position to be obtained without the know- 
ledge of the navigator. 

Navigation is the science of guiding a craft from one position to another by any 
chosen path. It includes the determination at any time of position, course bear- 
ing, etc. Various special navigational operations such as homing, flying fixed 
courses and vectoring is considered as limited or restricted navigation. 

A navigation aid is a device which provides the navigator with some or all of the 
following information: 

(a) present position 

(b) course heading 

(c) speed (ground or relative) 

(d) location of geographical surroundings 

(e) location of other craft in the vicinity 

(f) right-left steering directions or automatic steering control^ 

(g) altitude (not covered in this report) 

The phase aspect of two or more antennas refers to the relative phase relations 
existing, at a given point in space, between the electromagnetic fields produced 
at that point by radiation from the individual antennas. The reference point 
chosen is at a distance from the antenna array which is large compared to the 
dimensions of the array itself. The phase aspect will depend on the relative 
phases of the currents in the various antennas, the spacing between antennas, 
and the azimuth and elevation of the reference point with respect to the array. 


^ This appears to be necessary on account of the diversity of meaning of many new 
navigational and radar words. 

+ The "navigator" may in this case be a robot. 


32.02 


Appendix A (cont* d) 


As an example, consider two antennas driven in phase and separated by a dis- 
tance equivalent to b electrical degrees at the frequency of operation. The phase 
aspect referred to a distant point, whose elevation angle is <f> degrees and whose 
azimuth angle is 0 (measured from the perpendicular bisector of the base line 
of the array), will then be b cos 0 sin 6 electrical degrees. 

The precision of a measurement is the numerical measure of its reliability, making 
allowance for all known errors and uncertainties. (The true value is not actually 
known.) 

A racon (RAdar beaCON) is a radio beacon employing radio pulse signals. 

The word radar is used in this report to describe equipment with which a distance 
is measured by recording the time taken by an electromagnetic disturbance to 
travel from one point to another and return. The returning disturbance may be 
a simple echo or a beacon response. Radar measurements are not necessarily 
restricted to pulse transmissions. 

A radio beacon is a radio signal station. Radio beacons are used for determination 
of azimuth and range, or for identification. 

Range means distance. This is common usage among ordnance, gunnery and radar 
personnel. The word range has also been used to designate a line defined by two 
fixed landmarks such as lighthouses or other easily visible markers. The word 
is used with this connotation in the expression radio- "range" to mean a line de- 
fined by radio signals from an antenna array. Whenever the word is used in this 
latter sense in the present report, attention is called to the usage. 

Relative bearing is an azimuth angle measured clockwise from above from any ar- 
bitrary reference direction, as for instance the craft heading. 

A responder beacon is a pulse-type receiver-transmitter used to receive an inter- 
rogating signal and to transmit automatically an identifiable reply signal. Re- 
sponder beacons used for IFF purposes are commonly referred to as transpond- 
ers. 

A responser is a receiver used to accept the reply from responder beacons. 

The word synchro is used here as a generic name for all such devices, including 
those having other names such as selsyn, autosyn, magnesyn, etc. 


Appendix B 


33.01 


Probable values of some physical and geodesic constants 

Velocity of electromagnetic waves in vacuum Vq = 2.99778 x 10® meters per second. 

Refractive index of air at standard temperature and pressure n = 1.000294 

Velocity of electromagnetic waves in air (S.T.P.)v = 2.99690 x 10® meters per second 
= 186,218 statute miles per second 
= 161,711 nautical miles per second 


1 nautical mile = 1.15155 statute miles 


Microseconds per statute mile 
Microseconds per loop statute mile 
Microseconds per nautical mile 

Microseconds per loop nautical mile 
Frequency for X = 2 statute miles 
Frequency for X = 2 nautical miles 
One microsecond is equivalent to 


= 10 ^ . 
186,218 

= 10.7401 


5.3700 


10 


6 


= 6.1839 


161,711 
= 12.3678 
= 93,109 cps 
= 80,850 cps 

299.7 meters 
or 0.1862 statute miles 
or 328 yards 


or 984 feet 

or 0.1617 nautical miles 

1® azimuth at 100 miles 

_ 

1.75 miles 

at 1000 miles 

= 

17.5 miles 

1 mile at 100 miles 

= 

0.5730 azimuth 

1 minute of azimuth at 100 miles 

= 

158.5 feet 

Equatorial radius of earth 

= 

3963.34 statute miles 

Polar radius of earth 

= 

3949.99 statute miles 

Radius of a sphere having the same volume as the earth = 

3958.89 statute miles 

1® latitude along a meridian 

= 

about 69.1 miles 


= 

10 longitude at the equator 

1000 statute miles along a meridian 

= 

about 14.5® latitude 

Distance d along a parallel of latitude for 1® change of longitude: 

Latitude d 

Latitude 

d 

(statute miles) 


(statute miles) 

0° 69.1 

50° 

44.4 

IQO 68.0 

60° 

34.6 

20° 64.9 

70° 

23.6 

30° 59.8 

80° 

12.0 

400 52.9 

90° 

0 


LIST OF MICROFILMED REPORTS 

DIVISION 13 •VOLUME 2-B 


1941 

Ml 


1942 

Ml 


M2 


M3 


M4 


1943 

Ml 


M2 


M3 


M4 


M5 


M6 


Notes (and diagrams] for pilots, observers and 
special W/T operators on enemy radio aids to 
navigation and accurate bombing under blind 
conditions. (n. a.) [Royal Air Force] 
Headquarters, No. 80 Wing (Great Britain]. 
July 21, 1941. 


How GEE works. (Report No. CS-13530- 
Tels-IA.) (n. a.) OSRD Liaison Of- 

fice No. WA-116-36. Air Ministry [Great 
Britain], May, 1942. 

Block diagram. Indicator timer and receiver. 
(Drawing No. A-2693.) A. Fringelin. 
MIT, Radiation Laboratory. October 16, 
1942. 

Flight tests over Bermuda. (Division 11. 
Loran Memorandum No. 116.) Fletcher 
Watson. (MIT, Radiation Laboratory.] 

(November 18, 1942.] 

Three-line fixes. (Division 11. Loran Mem- 
orandum No. 122.) Fletcher Watson. 
[MIT, Radiation Laboratory.] December 

15, 1942. 


Service areas of loran pairs and chains. (Di- 
vision 11. Loran Report No. 28.) J. A. 
Pierce. MIT, Radiation Laboratory. 
March 6, 1943. 

Determination of errors in the loran system. 
(Division 11. Loran Report No. 26.) 
Donald G. Fink. MIT, Radiation Labora- 
tory. April 6, 1943. 

Some ionospheric notes on SS loran proposals. 
Edward V. Appleton, R. Naismith and W. R. 
Piggott. OSRD Liaison Office No. WA- 
993-2a. Operations and Technical Radio 

Committee, Sub-Committee on SS Loran [Great 
Britain]. August 6, 1943. 

Comparison of vector and dot-dash methods in 
the Oboe steering problems. (Division 14. 
Report No. 63.) A. C. Hughes. MIT, 
Radiation Laboratory. September 18, 

1943. 

Examination of SW-GKE chain. (Report No. 
CCDU-43/65.) R.F.B. Stead. Royal 
Air Force, Coastal Command Development Unit 
[Great Britain]. November 4, 1943. 
Rebecca and Eureka equipment, Australian. 
(Report No. RP-193.) E. B. Mulholland. 
OSRD Liaison Office No. II-5-5724. Council 
for Scientific and Industrial Research, Radio- 
physics Laboratory [Australia]. December 

7, 1943. 


M7 Radio and radar. SS loran. (Report No. 
NAR;X-5016.) (n. a.) [US Navy 

Department, Intelligence Division, Great 
Britain.] December 17, 1943. 


Ml Some notes on 2-mc loran propagation. (Divi- 
sion 11. Loran Memorandum No. 134.) 
David Davidson. [MIT, Radiation Labo- 

ratory.] January 1, 1944. 

M2 Optimum band width for loran receivers. (Divi- 
sion 11. Loran Memorandum No. 137.) 
David Davidson. [MIT, Radiation Labo- 
ratory.] January 27, 1944. 

M3 Index of loran reports and instruction manuals. 
(Division 11. Loran Memorandum No. 138.) 
David Davidson. MIT, Radiation Labo- 
ratory. February 10, 1944. 

M4 Consol range and accuracy trials. (Report No. 
CCDU-44/13.) R. F. Anstead. Royal 

Air Force, Coastal Command Development Unit 
[Great Britain]. February 26, 1944. 

M5 Elements of loran. (Division 14. Report 

No. 499.) B. W. Sitterly. OEMsr- 

262. MIT, Radiation Laboratory. 
March 8, 1944. 

M6 Radio set SCR-584, service. Theory, trouble- 
shooting and repair, with parts list. (Technical 
Manual No. TM-1 1-1524.) (n. a.) 

US War Department. March 10, 

1944. 

M7 Trials of loran interference with port wave W/T 
and R/T communication [On] HMS Scott [during 
the period from] February 15 [tO] 22, 1944. 
H. V. Scott. Admiralty Signal Establish- 

ment [Great Britain]. March 14, 1944. 

M8 Precise navigation by means of a radar map 
superposed on the plan position indicator. 
(Division 14. Report No. 503.) Ed- 
ward E. Miller and D. B. McLaughlin. 
OEMsr-262. MIT, Radiation Laboratory. 

April 7, 1944. 

M9 [Chart on] German bombing and navigational 
aids. (n. a.) [May, 1944.] 

MIO Revised notes on Sonne. (Report No. S-3000/ 
19/Sigs.) (n. a.) Royal Air Force, 

Headquarters, No. 80 Wing [Great Britain]. 
May 16, 1944. 

Mil Video stretching as a method for improving 
X-band beacon reception. (Division 14. Re- 
port No. 91.) T. H. Waterman and S. D. 
Bennett. [MIT, Radiation Laboratory.] 

May 26, 1944. 

M12 Siting and range of microwave beacons. (Divi- 
sion 14. Report No. 590.) W. M. 


^CONFIDENTIAL 


34.01 


34.02 


LIST OF MICROFILMED REPORTS 


M13 

M14 

M15 

M16 

M17 

M18 

M19 

M20 

M21 

M22 

M23 

M24 

M25 

M26 


Preston. OEMsr-262. MIT, Radia- 
tion Laboratory. July 5, 1944. 

Principles of radar. Edward W. Kimbark, 
John C. Batchelor and others. MIT, Ra- 
dar School. September, 1944. 

GEE-H airborne equipment, ARI-5525. 
(Report No. CD-0808A(2).) (n. a.) 

OSRD Liaison Office No. WA-3296-11. Air 
Ministry (Great Britain]. September, 

1944. 

Second report on experimental studies of iono- 
spheric propagation as applied to the loran 
system. (Report No. R-7.) (n. a.) 

National Bureau of Standards, Interservice Ra- 
dio Propagation Laboratory. October 11, 

1944. 

[Translations ofi] BHF Rundschreiben Nos. 
3, 4 and 5 [byj Straimer. (n. a.) October 
21, 1944. 

Consol as an aid to navigation. (Report 
No. CC/S-9005/12/8/NAy.) N. Ward. 
Royal Air Force, Coastal Command, Head- 
quarters [Great Britain]. October 28, 1944. 

Radar transmitter. Type T-1365. (Report No. 
CD-0895-D.) Harold Scott. Air 
Ministry [Great Britain] . November, 1944. 

Trials of loran SS chain. (Part HI. Report 
No. BDU-59.) P. Speare. Royal Air 
Force, Bombing Development Unit [Great 
Britain]. November 1, 1944. 

Instructions for plotting radio fixes by the GAF 
radio-beacons Elektra-Sonne. Translation of : 
Merkblatt ftir die praktische auswertung von 
funkortungen nach luftfunkfeuer Elektra-Sonne. 
(Reg. No. P/5772/NID.) (n. a.) 

(Great Britain.] November 14, 1944. 
Letter to Dr. L. A. DuBridge. Subject: 
German Sonne system. J. A. Pierce. 
November 28, 1944. 

Decca navigator. (n. a.) [Decca 

Radio and Television, Ltd., Great Britain.] 
[December, 1944.] 

[Radar] GEE-R, ARI-5342. (Report No. CD- 
0808-D.) Harold Scott. Air Minis- 

try [Great Britain]. December, 1944. 

Preliminary analysis of radar navigation aids. 
(Engineering Memorandum No. PEM-16C.) 
P. J. Herbst. Radio Corporation of 
America. December 1, 1944. 

Communication radio and radar. Investiga- 
tion of Sonne and Decca navigational aid. 
(n. a.) US Navy Department, Intelligence 
Division (Great Britain]. December 14, 
1944. 

Investigation of German Sonne navigational air 
radio station. (Alsos No. 42.) E. N. 
Dingley, Jr. OSRD Liaison Office No. 

WA-4312-3. SHAEF, Combined Intelli- 


M27 

M28 

1945 

Ml 

M2 

M3 

M4 

M5 

M6 

M7 

M8 

M9 

MIO 

Mil 


CcoSttoential 


gence Objectives Sub-Committee. Decem- 
ber 14, 1944. 

Radio set AN /CPS- 1 service manual. Theory, 
trouble-shooting and repair. (Technical Manual 
No. TM-11-1544.) (n. a.) US War 

Department. December 15, 1944. 
Comparative survey of long distance radio navi- 
gational aids. (Paper No. 2.) Observa- 

tions on the German Sonne system. (Paper No. 
3.) C. G. Phillips. October 6, 1944. 


Theoretical comparison of the two- and three- 
aerial Sonne systems. (Technical Note No. 
RAD-263.) A. H. Brown. OSRD 
Liaison Office No. WA-3844-1. ' Royal Air- 

craft Establishment (Great Britain]. Jan- 
uary, 1945. 

An investigation to find a suitable method of 
checking the stability of the position line given 
by Sonne. (Radio Report No. 1273.) H. T. 
Mitchell, T. Pilling and K. H. Ferguson. 
OSRD Liaison Office No. WA-4099-3. Post 
Office Engineering Department [Great Britain] . 
January 14, 1945. 

Sonne. (Report No. S-935-17, Serial 00664.) 
E. N. Dingley, Jr. US Navy Department, 
BuShips, Radio Division. January 16, 
1945. 

Handbook of operating instructions for radio set 
AN/APN-4. (Report No. AN-08-30 APN4-2.) 
(n. a.) US War Department, US Navy 
Department and Air Council [Great Britain]. 
Revised: January 21, 1945. 

A microfilm chart projector for radar navigation. 
(Division 14. Report No. 658.) D. B. 
McLaughlin and C. A. Smith. OEMsr- 
262. MIT, Radiation Laboratory. 
January 23, 1945. 

Loran tests on 8th Air Force heavy weather re- 
connaissance missions. R. Dorr. [Feb- 
ruary (?) 1945.] 

Lecture notes [on] SS loran. (Aids to Navi- 
gation Memorandum No. 7.) George H. 
Trow. Coastal Command [Great Bri- 
tain]. February, 1945. 

German night fighter electronic aids. Notes on 
enemy aircraft and armament. (n. a.) 

[February, 1945.] 

The statistics of beacon interrogation. (Divi- 
sion 14. Report No. 602.) H. H. 

Bailey. OEMsr-262. MIT, Radia- 
tion Laboratory. February 5, 1945. 
Three-path rotating lighthouse system for air- 
port control. (Proposal No. 287.) (n. a.) 

Federal Telephone and Radio Corporation. 
F'ebruary 6, 1945. 

Recommended operational procedure for use of 


LIST OF MICROFILMED REPORTS 


34.03 


Nosmo over a complex target. (Division 14. 
Report No. 63.) R. N. Close. MIT, 
Radiation Laboratory. February 17, 1945. 
M12 Radar television system of air navigation. 
(Engineering Memorandum No. PEM-17C.) 
P. J. Herbst. Radio Corporation of 
America. February 17, 1945. 

M13 Trials of GH, Mark II. (Part I. Report 
No. BDU-65.) P. Speare. Royal Air 
Force, Bombing Development Unit [Great 
Britain]. February 27, 1945. 

M14 Atlantic loran research flight [ofj October, 1944. 
(n. a.) US Navy Department, Aviation 
Training Division. March, 1945. 

M15 Letter to The Under-Secretary of State, Air 
Ministry. Subject: Proposals for the 

European and Eastern Atlantic SS loran sys- 
tems. R. H. Woodward. OSRD 
Liaison Office No. WA-4052-5. British 
Branch Radiation Laboratory [Great Britain]. 
March 8, 1945. 

M16 European and eastern Atlantic SS loran chain. 
(Report No. CMS-107/R-3c.) (n. a.) 

Air Ministry [Great Britain]. March 26, 

1945. 

M17 Pocket handbook of airborne loran electronic 
navigation. Receivers and indicators, 

(n. a.) US Navy Department, Aviation 

Training Division. April, 1945. 

M18 US radar survey. Section 2, Shipborne radar. 
(Division 14.) (n. a.) April 1, 1945. 

M19 Loran operational tests aboard USS Plunkett, 
DD 43 I [from] 16 February to 21 February, 1945. 
(Report No. M-935-1570b.) Robert L. 

Frank. US Navy Department, BuShips. 

April 2, 1945. 

M20 Handbook of maintenance instructions for 
radio set AN/APN-4. (Report. No. AN-16- 
30APN4-3.) (n. a.) US War De- 

partment, US Navy Department and Air 
Council [Great Britain]. April 14, 1945. 

M21 Benito control of fightei’s and Egon control. 
(Report No. 33.) (n. a.) OSRD 

Liaison Office No. WA-4327-3(2). Air 
Ministry, Air Scientific Intelligence [Great 
Britain]. April 20, 1945. 

M22 Trials of loran in Mosquito VI and Oxford air- 
craft. (Report No. 34.) (n. a.) Royal 


M23 

M24 

M25 

M26 

M27 

M28 

M29 


NO DATE 
Ml 


M2 


M3 


M4 


Air Force, Central Fighter Establishment [Great 
Britain]. May 29, 1945. 

The Sonne (Consol) navigation system. (Re- 
port No. 9.) Max I. Rothman. Wat- 
son Laboratories. June, 1945. 

US radar survey. Section 4, Navigational 
radar. (Division 14.) (n. a.) June 15, 1945. 

Suggested improvements for loran system. 
Problem [No.] S752R-C. (Report No. C-S67- 
9/10-357.) C. A. MiUer, Jr. US 
Naval Research Laboratory. June 25, 

1945. 

Radar. ([Issue] No. 10.) (n. a.) US 

Army Air Forces, Office of the Air Communi- 
cations Officer. June 30, 1945. 
Elektra-Sonne. Translation of Lorenz’ descrip- 
tion and operating instructions for Sonne 8 HF, 
Rack 111. (Report No. 10.) (n. a.) 

Air Ministry [Great Britain]. July 17, 

1945. 

Radio and radar equipment in the Luftwaffe. 
([Part] II. Report No. 357.) S. D. 
Felkin. OSRD Liaison Office No. WA- 

5076-3. Assistant Director of Intelligence 

(K) [Great Britain] and US Air Interrogation. 
July 25, 1945. 

The future of hyperbolic navigation. (Division 
14. Report No. 625.) J. A. Pierce. 
OEMsr-262; Service Project Nos. SC-56 and 

AN-18. MIT, Radiation Laboratory. 

August 18, 1945. 


Proposed antenna design for low-frequency 
loran. (Report No. ORS-P-22-2.) E. M. 
Johnson and Carl E. Smith. [US Ai-my 

Signal CorpS] Operational Research Staff, 
(n. d.) 

Notes on Sonne navigation beacon system. 
(Technical Minute No. RAD-60.) C. Wil- 
liams and C. D. Smith. [Royal Aircraft 

Establishment, Great Britain.] (n. d.) 

Universal communication, airport control, traffic 
control and aerial navigation system. Part 
III, Aerial navigation. • (Proposal No. 235.) 
(n. a.) Federal Telephone and Radio 
Corporation. (n. d.) 

Sonne [diagram, with translation of German 
technical terms]. (n. a.) (n. d.) 


CONFIDENTIAL 




i^i 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS 


Contract Number 

Name and Address of Contractor 

Subject 

OEMsr-1441 

Central Communications Research, 

Cruft Laboratory, Harvard University, 
Cambridge, Mass. 

Analysis of Available Methods of Navigation 


CONFIDENTIAL 


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SERVICE PROJECT NUMBERS 

The projects listed below were transmitted to the Executive Secretary, 
National Defense Research Committee [NDRC], from the War and 
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Service Project Number 

Subject 


AN-31 

Analysis of Available Methods of Navigation 



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INDEX 


Air traffic control system (radar), 27.01 
Airborne Bups, 2,19 
Airborne radar for navigation, 22.01- 
22.30, 31.01; see also Radar 
beacons. Radar mapping 
Aircraft direction-finders, automatic, 

16.01-16.05, 31.06 
Alford loops, 19.03, 19.05 
Amplifier; D. C., 8.06 
delay trigger, 22.21 
gated, 22.21 
phase-stable, 13.05 

A-N radio “range,” 1.22, 15.01-15.06, 

31.06 

AN/APA-9 (Aspen), 2.19 
AN/APA-40 (Micro H Mark II), 5.01 
AN/APA-44 (GPI), 29.01-29.08, 31.06 
AN/APA-46 (Nosmo), 22.29 
AN/APN-2 (interrogator), 2.18 
AN/APN-2A (interrogator or trans- 
ponder), 2.18 

AN/APN-2Y (interrogator), 2.18 
AN/APN-19 (Rosebud), 2.19 
AN/APN-29 (Rosebups), 2.19 
AN/APN-34 (short-range approach), 

24.01, 31.06 

AN /APQ-7 (search radar), 29.01 
AN/APQ-13 (H2X), 22.01, 22.29 
AN /APQ-34 (search radar), 29.01 
AN/APS-15 (H2X), 5.01, 22.01-22.30, 

31.06 

accuracy, 22.01 
antenna, 22.08 
A-scope, 22.05 

delayed sweep for beacon observa- 
tion, 22.20-22.21 

flux-gate compass system, 22.08- 

22.10 

functioning of system, 22.21-22.28 
Nosmo attachment, 22.29-22.30 
phantastron delay circuits, 22.17- 

22.20 

receiver components, 22.10-22.11 
RF components, 22.06-22.08 
sweep circuits, 22.05-22.06 
timing circuits, 22.11-22.17 
AN/CPN-3 (responder beacon), 2.19 
AN/CPN-6 (BGX), 2.08-2.18 
antennas, 2.10 
codes, 2.15-2.17 
IF amphfier, 2.10-2.14 
oscillator, 2.10-2.14 
pulse-width discrimination circuit, 
2.14-2.15 

receiver components, 2.13 
RF lines, 2.10 

transmitter components, 2.17-2.18 
AN/CPN-7 (BABS), 2.19 
AN/CPN-8 (BPS), 2.18 
AN/CPN-13 (transponder beacon), 

2.20 

AN /CPN-15 (transponder beacon), 

2.20 

AN/CPS-1 (MEW), 23.01-23.02 
AN/PPN-1 (responder beacon), 2.19 


AN/PPN-8 (transponder beacon), 2.20 
Anterma array, transmitting, 1.41 
Antenna pattern for azimuth naviga- 
tion systems, 1.15 

Anti-aircraft gun-laying radar (AGL), 
23.04 

Anti-hunt devices, 7.06 
AN/UPN-1 (BUPS-D.C.), 2.19 
AN/ JPN-2 (BUPS-A.C.), 2.20 
AN/UPN-3 (BUPX-A.C.), 2.20 
AN/UPN-4 (BUPX-D.C.), 2.20 
A.R.L. intermittent phase-comparison 
distance-measuring system, 6.01- 
6.04, 31.06 

A.R.L. one-shot distance-measuring 
system, 7.01, 31.06 
A-scope, 22.05 

.\spen (AN/APA-9), 2.19, 3.04 
Aural presentation of navigational in- 
formation, 1.37-1.38 
Aural VHF “ranges”, 15.03-15.05 
Autofocus Microfilm Projector, 26.01 
Automatic aircraft direction finders, 

16.01- 16.05, 31.06 

Azimuth navigation systems; aircraft 
direction-finders and homing 
systems, 16.01-16.05 
AN Radio “Range”, 15.01-15.06, 

31.06 

Bendix automatic position plotter, 

18.01- 18.06 

CAA LF omnidirectional beacon, 

20.01 

CAA VHF omnidirectional beacon, 

19.01- 19.10 

CAA VHF radio “ranges”, 15.03- 

15.06 

Electra, 30.01 

Federal long-range system, 21.01- 
21.08 

Knickebein, 30.01-30.02 
Sonne, 17.01-17.26 

Azimuthal navigation systems, general 
discussion, 1.13-1.17 

BABS (AN/CPN-7), 2.19 
Balloons for carrying radio antennas, 
1.41 

Bandwidth requirements for electronic 
navigation systems, 1.41-1.51 
Beacon (AN/CPN-6) 2.08-2.18 
Beacon codes, 2.03-2.04 
Beacon measurements, accuracy of, 
2.05 

Beacon response signal, 2.04 
Beacons and interrogators; see Radar 
beacons and interrogators 
Bendix Automatic Position-Plotter, 

18.01- 18.06, 31.06 
Benito, 30.01 

Bernhard-Bemhardine, 30.04 
BGX (AN/CPN-6), 2.08-2.18 
Black Maria, 2.19 

Blind bombing systems; Knickebein 
30.01 


Micro H, 5.01 
Oboe, 3.01 
Ruffian, 30.02 
Shoran, 4.01 

Blind landing system (radar), 27.01 
Blind navigation with H2X, 22.02 
Bomber-control navigational system, 
(Ruebezahl), 30.03 

Bombing from high altitude with H2X, 

22.03 

Bombsight, Norden, 22.29 
BPP (AN/PPN-2), 2.19 
BPS (AN/CPN-8), 2.18 
British airborne interrogators, 2.18 
British “Penwiper” receiver, 3.05 
British “Pepperbox” receiver, 3.05 
British responder beacons, 2.18 
British Sonne system (Consol), 17.02 
BUPS (AC) (AN/UPN-2), 2.20 
BUPS (DC) (AN/UPN-1), 2.19 
BUPX (AC) (AN/UPN-3), 2.20 
BUPX (DC) (AN/UPN-4), 2.20 

CAA LF Omnidirectional Beacon, 

20.01, 31.06 

CA.^ VHF Omnidirectional Beacon, 

19.01, 31.06 

CAA VHF radio range, 15.03 
Canadian distance-measuring system, 

8.01-8.09, 31.06 
Cat and mouse stations, 3.01 
Circle blanking, 4.09 
Circuits, electronic; amplifier, D. C., 

8.06 

amplifier, delayed trigger, 22.21 
amplifier, gated, 22.21 
amplifier, range-mark, 22.13 
computing circuits for position plot- 
ter, 18.05 

counter circuits for frequency di- 
vision, 12.13 
CRO sweep, 12.23 
Eccles-Jordan trigger circuit, 12.20 
frequency divider, regenerative type, 
4.06-4.07 

gate generator, 22.21 
Hartley circuit, 11.08 
Miller Rundown, 8.03 
multivibrators, 12.20 
phantastron delay circuits, 5.02, 
22.17-22.20 

phase comparison indicator, 14.08, 
19.09 

phase discriminating rectifier cir- 
cuits, 13.05 

phase locking circuit, 13.05 

phase meters, 13.03, 13.06 

phase sensitive motor control circuit, 

6.03 

phase shifters, 13.07, 17.17 
pulse coding, 2.16 
pulse selectors, 4.07 
pulse shaping circuits, 2.17, 22.07 
pulse width discriminator circuit, 
2.14-2.15 


CONFIDENTIAL 


37.01 


37.02 


INDEX 


range- tracking, automatic, 8.01 
rectangular to polar coordinate re- 
solver circuit, 29.06 
sawtooth voltage generator, (pre- 
cision), 8.04, 22.05 
scaling circuits, 12.13 
side-frequency generator, 19.04 
“snap circuit,” 8.01, 8.03, 8.05 
sub-carrier generator, 19.04 
timing circuits, crystal controlled, 
22.16 

Circular lines of position, 1.05 
Civil aviation, use of A-N radio range, 
15.01 

Code formation in beacon AN /CPN-6, 
2.15-2.17 

Coding circuit for pulses, 2.16 
Compass system, flux-gate, 22.08-22.10 
Composite navigation systems, 1.17 
Computer, GPI, 29.01-29.08 
Computing circuits (Bendix Automatic 
Position-Plotter), 18.05 
Consol, (British Sonne system), 17.02 
Continuous-wave navigation systems, 
1.30 

Corner reflectors, 26.06 
Counter circuits for frequency division, 
12.13 

Coverage area of navigation system, 
factors affecting, 1.17-1.18 
Cube-law CRO sweep circuit, 25.03 
“Cycle matching”, 12.09 
Cyclop, 30.04 

“Dash” pattern, 17.04 
D. C. amplifier, 8.06 
Decca Navigational System, 13.01- 
13.08, 31.06 

equipment required, 13.03 
phase meter, 13.05 
receiver-indicator, 13.05-13.06 
slave station, 13.06 
Delay line, liquid, 23.03 
Differential range navigation systems; 
see Hyperbolic navigation sys- 
tems 

Diffraction of radio waves, 1.23 
Direction-finders, automatic, aircraft, 

16.01-16.05, 31.06 

Directivity patterns, 15.01, 16.01, 17.02 
Diskus, 30.05 

Distance measurement navigation sys- 
tem; see Range navigation sys- 
tems 

Distance meter, intermittent phase- 
comparison type, 6.02 
Doppler shifts in MTI, 23.03 
Dora, 30.04 

Drift and rate stations, 3.01 
“Ducting” of radiowave, 1.23 

“E” layer in ionosphere, 1.26 
Eccles-Jordan trigger circuit, 12.20 
Egon, 30.03 

Electronic index for the PPI, 29.01 
Electronic navigation systems; see 
Navigation systems, electronic 
Elektra, 17.05, 30.01 


Enemy navigational systems, 30.01- 

30.06 

Equisignal, 17.05 
Erika, 30.05 

Error distribution in Loran readings, 
12.09 
Eureka, 2.18 

“F” layer in ionosphere, 1.26 
Federal airport traffic control system, 

25.01- 25.05, 31.06 

rotating hghthouse system, 25.02- 

25.05 

3PR system, 25.01-25.02 
Federal long-range navigation system, 

21.01- 21.08, 31.06 
accuracy, 21.08 
azimuth unit, 21.07 
directivity patterns, 21.01-21.08 
receiver-indicator, 21.05 

Film processing in 10 seconds, 23.03 
Flux-gate compass system, 22.08-22.10 
Frequency band allotments for naviga- 
tion systems, 31.05 

Frequency divider, regenerative type, 
4.06-4.07 

Frequency multiplier Klystron, 28.01 
Frequency requirements for electronic 
navigation systems, 1.41-1.51 
Freya radars, 30.04 

G system of navigation, see Gee navi- 
gation system 
Gap coding, 2.03 
Gate generator, 22.21 
Gating pulses, 11.14 
GCA (ground-controlled approach) 
radar, 27.05 

GE random interrogation distance- 
measuring system, 9.01-9.02, 

31.06 

GE time rationing distance-measuring 
system, 10.01-10.02, 31.06 
Gee navigation system, 11.01-11.15, 

31.06 

accuracy, 11.01 
indicator, 11.08-11.11 
indicator circuits, 11.12-11.15 
obtaining a fix, 11.10-11.12 
principle of operation, 11.02-11.05 
range, 11.01 
receiver, 11.08-11.10 
time measurement, 11.10 
timing circuits, 11.05, 11.12-11.15 
transmitter, 11.05-11.08 
Generators; gate, 22.21 
rotating capacitor, 19.05 
sawtooth voltage (precision), 8.04, 

22.05 

side-frequency, 19.04 
square-wave pedestal, 12.21, 12.23 
sub-carrier, 19.04 
sweep voltage (CRO), 12.23 
German navigation systems, 30.01- 

30.06 

Benito, 30.01 

Bernard-Bernardine, 30.04 
Cyclop, 30.04 


Diskus, 30.05 
Dora, 30.04-30.05 
Egon, 30.03 
Elektra, 17.05, 30.01 
Erika, 30.05 
Hermine, 30.02-30.03 
Hyperbel, 30.04 
Hyperbol, 30.04 
Knickebein, 30.01 
Nachtfee, 30.05 
New Erika, 30.05 
Ruebezahl, 30.03-30.04 
Ruffian, 30.02, 30.03 
Schwanboje, 30.05 

Sonne, 1.42, 17.01,17.10,17.15-17.18 
Zyklop, 30.04 

German phase-shifting circuits, 17.17 
Glide-path radar system, 27.05 
GPI (Ground Position Indicator), 
22.30, 29.01-29.08, 31.06 
azimuth marker system, 29.06 
equipment, 29.01 
“memory-point tracking,” 29.05 
principles of operation, 29.02 
range marker, 29.07 
wind data, 29.03 
Ground clutter, 23.03 
Ground radar, 31.02, 31.06 
Ground wave propagation, 1.23 
Ground waves, separation from sky 
waves, 1.27 

Gun-laying radar, antiaircraft, 23.04 
Gyro flux-gate compass system, 22.08- 
22.10 

H navigation systems; A.R.L. one 
shot distance-measuring sys- 
tem, 7.01-7.06 

Canadian distance-measuring, 8.01- 
8.09 

Oboe, 3.01-3.05 
Shoraii, 4.01-4.13 
Hartley circuit, 11.08 
Hermine, 30.02 

High-altitude bombing with H2X, 22.03 
Homing, 1.38 

Homing systems, 16.01-16.05 
Horizon-range formula, 2.05 
H2X equipment; see AN/APS-15 
H3X (blind bombing, radar), 22.29 
Hyperbol (or Hyperbel), 30.04 
Hyperbolic navigation systems; Decca, 

13.01-13.08 
Gee, 11.01-11.15 
H3X, 22.29 
Loran, 12.01-12.29 
POPI, 14.01-14.12 

Hyperbolic navigation systems, general 
discussion, 1.06-1.13 
Hyperbolic position lines, 1.08, 14.04 
Hyperbolic sweep (CRO), 31.02 

IFF transponder beacons, 2.20 
Indicator ID-6B/APN4, 12.13-12.26 
displays and sweep speeds, 12.26 
general principles, 12.13 
operation of the left-right switch, 
12.20 


:^ONFIDENTIAL 


INDEX 


37.03 


I^edestal delay multivibrators, 12.20 
pedestal generator, 12.21 
pulse rate selection, 12.17-12.19 
sweep voltage generator, 12.23 
Integrating phase meter, 13.03 
Interrogatoi-s; see Radar beacons and 
interrogators 
Ionospheric tilt, 1.30 

Knickebein, 30.01 

Landing operation navigational con- 
trol, 26.04 

LCC (Landing Craft, Control), 26.04 
LF Loran, 12.02, 12.08, 31.06 
LF radio “range,'’ 15.02 
Light, velocity of, 1.18 
Limacon directivity pattern, 19.01 
Long-range navigation systems; see 
Federal, Sonne, Standard Loran 
Loran, 1.22, 1.42, 12.01-12.29, 31.06; 
see also LF Loran, SS Loran, 
Standard Loran 
bandwidth requirements, 1.42 
coverage areas, 12.03-12.09 
errors, 12.09-12.12 
indicator, 12.13-12.26 
indicator waveforms, 12.22 
maximum range, 12.01 
principles of operation, 12.02-12.03 
pulse rates, 12.09 
receiver, 12.12-12.13 
stations, 12.03-12.09 
Low-frequency omnidirectional beacon, 
20.01 
L-scan, 2.03 
Lucero, 2.18 

Manual direction finder, 16.01 
Map-PPI superposition, 26.01-26.06, 
31.06 

advantages, 26.03 

XALOC system, 26.03-26.06, 31.06 
television techniques, 26.02 
Marker offsetting, 4.10 
Marker pulses, 4.09 
Meaconing, 1.39 

Mechanical indicator for navigation 
information, 1.38 
“Memory-point tracking”, 29.05 
MEW (Microwave early-warning ra- 
dar), 23.01-23.02, 31.06 
antenna system, 23.02 
components, 23.02 
ground clutter, 23.03 
indicating equipment, 23.02 
new methods of presentation, 23.03 
Micro-H, 5.01-5.04, 22.30, 31.06 
Micro-H Mark I, 5.01 
Micro-H Mark II, 5.01-5.04 
Microwave beacon AN/CPN-6; 2.05 
Microwave beacons, ranging and siting 
of, 2.05-2.08 

Microwave early-warning radar 
(MEW), 23.01, 23.02, 31.06 
Microwave Oboe ground station, 3.05 
Miller Rundown circuit, 8.03 


Motor-control circuit, phase sensitive, 
6.03 

MTI (moving target indication), 23.03 
MTI modification kit, 23.05 
Multivibrators, pedestal delay, 12.20 

Nachtfee, 30.05 

NALOC system (Navigational aids to 
Landing Operations Commit- 
tee), 26.03-26.06 

Navigation by airborne radar, 22.30 
Navigation by search radar, 23.01 
Navigation by means of sonic buoys, 
26.04 

Navigation of fighter craft, 1.37 
Navigation requirements, coordination 
of, 31.08 

Navigation systems, electronic; {see 
p. 31.06 for listing of types) 
accuracy of, 1.18-1.34, 31.02-31.03 
comparison of, 31.01-31.11 
coverage, 1.17 

equipment required, 1.40-1.41 
errors due to ambiguities of position, 
1.30-1.33 

frequency and bandwidth, 1.41-1.51, 
31.04, 31.05 

future requirements, 31.08-31.11 
operating skill required, 1.39-1.40 
propagation path, 1.23-1.30 
recommendations for futher research, 
31.11 

sky-wave erroi*s, 31.03-31.04 
sources of error, 1.18 
time measurement, 1.20-1.23 
transmitter site considerations, 31.04 
type of presentation, 1.34-1.39 
types of systems, general discussion, 
1.05-1.17 

Navigation with the aid of corner re- 
flectors, 26.06 
New Erika, 30.05 

NMP (Navigational Microfilm Pro- 
jector), 26.01, 26.04 
Norden bombsight, 22.29 
Norton's Formula, 1.23 
Nosmo attachment for AN/APS-15, 
22.29-22.30 

computing equipment, 22.30 
doppler effect, 22.30 
functions, 22.29 

Oboe navigation system, 3.01-3.05, 
31.06 

Aspen (airborne equipment), 3.04- 
3.05 

course determination, 3.01 
ground stations, 3.01, 3.05 
tracking aircraft, 3.05 
Omnidirectional beacons, 19.01-20.01 
One shot distance-measuring naviga- 
tion system, 7.01-7.06 
Oscilloscope indicators for navigational 
information, 1.38-1.39 
Overinterrogation, 2.04 

“Pedestals” in Loran system, 12.15 


: CONFIDENTIAL 


“Penwiper” receiver, 3.05 
“Pepperbox” receiver, 3.05 
Phantastron delay circuits, 5.02, 22.17- 
22.20 

Phase angle measurement devices, 1.21 
Phase comparison distance-meter, 6.04 
Phase comparison indicator, 14.08, 
19.09 

Phase discriminating rectifier circuit, 
13.05 

Phase locking circuit, 13.05 
Phase meter circuit, 13.06 
Phase modulated pulses, 3.02 
Phase sensitive motor control circuit, 

6.03 

Phase shifter, servo mechanism, 4.07, 
6.02, 14.03 

Phase shifter circuits, 13.07, 17.17 
Phase stable amplifier, 13.05 
Photographic processing in 10 seconds, 

23.03 

Photographic projection PPI (PT), 
23.03, 26.02 

Pilot direction indicator (PDI), 29.01 
POPI (Post-Office Position Indicator), 

14.01- 14.12, 31.06 
antenna spacing, 14.07 
equipment required, 14.01 
principles of operation, 14.01-14.04 
procedure for obtaining a fix, 14.12 
sector identification, 14.10-14.12 
spacing of position-lines, 14.04-14.07 

PPI operating circuits, 22.15 
PPI scope, 1.22, 22.01-22.30 
Projectors for Map-PPI superposition, 
26.01 

Propagation path for radio transmis- 
sions, 1.23 

Pulse coding circuits, 2.16 

Pulse doppler drift determination, 22.29 

Pulse forms, 1.42 

Pulse modulated navigation system, 
1.30 

Pulse selectors, 4.07 
Pulse shaping circuits, 2.17, 22.07 
Pulse width discriminator circuit, 2.14- 
2.15 

Pure range navigation systems; see 
Range navigation systems 

Racons; see Radar beacons and interro- 
gators 

Radar, airborne, 22.01-22.30 
Radar, value as a navigational aid, 

31.01- 31.02 

Radar beacons and interrogators, 2.01- 

2.21 

accuracy, 2.05 
AN/CPN-6; 2.08-2.18 
coding, 2.03-2.04 
interrogator-responsers, 2.18 
overinterrogation, 2.04-2.05 
ranging and siting, 2.05-2.08 
responders and transponders, 2.18- 
2.20 

response signals, 2.04 
triggering requirements, 2.01, 2.03 
uses, 2.01 


37.04 


INDEX 


Radar horizon range calculation, 2.05 
Radar mapping, 26.01-26.06 
Radial lines of position, 1.15 
Radial navigation systems, 1.13 
Radiation patterns, 15.01, 16.01, 17.02, 
19.02, 21.02 
Radio beacon, 2.01 
Radio pulse, shape of, 1.44 
Radio “range” navigation system, 1.38 
Range and azimuth composite navi- 
gation systems; AN/APS-15 
(H2X), 22.01-22.28 
Benito, 30.01 

Federal airport traffic-control ,25.01- 

25.05 

GPI, 29.01-29.08 

MAP-PPI superposition, 26.01-26.06 
MEW, 23.01-23.02 
RCA television-radar system, 27.01- 
27.07 

SCR 584, 23.03-23.05; 

Sperry omnidirectional range and 
distance indicator, 28.01-28.02 
Range and siting of microwave beacons, 

2.05 

Range, azimuth and track combination 
navigation system (AN /APN 
34), 24.01-24.02 
Range coding, 2.03, 23.01 
Range mark generator and amplifier, 

22.13 

Range navigation systems; A.R.L. 
intermittent phase-comparison 
distance-measuring system, 6.01- 
6.04 

A.R.L. one-shot distance-measuring 
system, 7.01-7.06 

Canadian distance-measuring sys- 
tem, 8.01-8.09 

GE random interrogation distance- 
measuring system, 9.01-9.02 
GE time-rationing distance-measur- 
ing system, 10.01-10.02 
H3X, 22.29 
Micro H, 5.01-5.04 
Oboe, 3.01-3.05 
Shoran, 4.01-4.13 

Range navigation systems, general 
discussion, 1.05-1.06 
Range-tracking circuit, automatic, 8.01 
R-9/APN4 (Loran receiver), 12.12- 

12.13 

“Rate” and “drift” stations, 4.02 
Rate-generator for anti-hunt control, 
16.03 

Rate-of-approach meter, 8.07 
RCA television-radar system, 27.01- 
27.07, 31.06 
altitude range, 27.03 
control in dense traffic, 27.04 
identification, 27.04 
principles of operation, 27.01-27.02 
use with blind approach, 27.04 
uses, 27.01 
Rebecca, 2.18 

Receiver biankg®cf9ASSIFIED 
RecommendattOTTr"for"fnTTl!ErresSTch 

31.11 


Rectifier circuits, phase discriminating, 

13.05 

Reflex-oscillator characteristics (Type 
723), 2.13 

Refraction of radio waves, 1.23, 4.03 
Resolver circuit, rectangular to polar 
coordinates, 29.06 
Responder beacons, 2.01, 2.18 
RLS (Rotating Lighthouse System), 
25.01 

Rooster beacon, 2.18 
Rosebud (AN/APN-19), 2.19 
Rosebups (AN/APN-29), 2.19 
Rotating lighthouse system, 25.01 
Ruebezahl, 30.03 
Ruffian, 30.02 


and tolerances, 
17.13-17.14, 


aid. 


of 


Sawtooth voltage generator (precision), 
8.04, 22.05 

Scaling circuits, 12.13 
Schwanboje beacons, 30.05 
SCR-521; 2.19 

SCR-584; 2.19, 23.03-23.05, 31.06 
accuracy, 23.04 
indicating equipment, 23.05 
uses, 23.04 
SCR-729; 2.18, 2.19 
Scrambling (in Shoran), 4.10 
Search radar as a navigational 
23.01-23.06 

Search radar system MEW, 23.02 
Sector ambiguity, 1.30 
Sector identification with POPI, 
14.10 

Self-orienting automatic direction- 
finder, 16.02, 16.04 
Sequence coding, 2.03 
Shoran, 1.22, 4.01-4.13, 31.06 
components, 4.06 
errors, 4.04 

performance, 4.11^.12 
principles, 4.01-4.03 
scrambling, 4.10-4.11 
Shoran distance, 4.03-4.04 
uses, 4.01 

Short-range approach navigation sys- 
tem, 24.01-24.02 
Side-frequency generator, 19.04 
Siting requirements for transmitters, 
31.04 

Sky wave, separation from ground 
wave, 1.27 

Sky-wave delay time, 1.26, 1.29 
Sky-wave errors, 31.03 
Sky-wave propagation, 1.23-1.29 
“Snaps” circuits, 8.01, 8.03, 8.05 
Sonic buoys, 26.04 
Sonne (Consol), 17.01-17.26, 31.06 
accuracy, 17.15 
bandwidth requirements, 1.42 
comparison of two-antenna and 
three-antenna systems, 17.25 
equisignal, 17.05 
errors, 17.15 

principle of operation, 17.02-17.10 
radiation pattern, 17.02-17.10 
sector ambiguity, 17.08 
three antenna systems, 17.10-17.13 
transmission sequence, 17.07 


transmitter errors 
17.20-17.25 

two antenna systems, 

17.16, 17.19-17.20 
Spencer Autofocus Microfilm Projector, 
26.02 

Sperry omnidirectional range and dis- 
tance indicator, 28.01-28.02, 

31.06 

Spinning-loop direction fiinder, 16.05 
SS Loran, 12.02, 12.06, 12.08, 12.10, 

31.06 

Standard Loran, 12.02, 12.03-12.07, 
12.09, 31.06 

Strobe sweep, 11.09-11.12 
Strobe timing pulses, 11.12 
Sub-carrier generator, 19.04 
Switched-cardioid homing system, 16.01 


Television-radar navigation system, 
27.01-27.07 

Three-antenna Sonne, 17.10, 17.21- 
17.24 

Three-path radar, 25.01 
3PR system, 25.01 

Time measurement with electronic 
navigation system, 1.20-1.23 
Tracking and release stations, 3.01 
Transmitting antenna array, 1.41, 19.03 
Transponders, 2.01, 2.18 
Trigger circuit, Eccles-Jordan, 12.20 
Trigger requirements of radar beacons, 
2.01, 2.03 
Truhe, 30.04 

Two-antenna Sonne, 17.13, 17.16, 

17.24-17.25 


Underwater-sound direction receiver 
(QBG) sound head), 26.04 


Velocity modulated (Type 723) tube 
characteristics, 2.13 
Velocity of light, 1.18 
VHF navigation systems; CAA VHF 
radio “ranges,” 15.03-15.06 
Hermine, 30.02 

VHF omnidirectional-beacon, 15.03 
VHF radio “range”, 15.02 
Video gates, 8.03 
“Video stretching”, 2.04 
Visual VHF “ranges”, 15.03-15.06 
VPR (Virtual PPI Reflectoscope), 
26.01, 26.04 


Wave propagation in the ionosphere, 
1.24 

Wave propagation over sea water, 1.24 
Width-modulated pulses, 3.04 


X-band radar; AN/APS-15, 22.01- 
22.28 

H3X, 22.29 

X-band radar responder beacon, 2.08, 
2.18 


YH (responder beacon), 2.19 
YJ (responder beacon), 2.19 


SEP 2 3 1960 





Defense memo 2 August 1960 


Zyklop, 30.04 

before servicing 


LIBRARY OF CONGRESS 





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